Great News! My dulcimer is really fixed. I found someone in North Georgia who actually cleaned up several frets, had to level one fret and built up the bridge under the bass string. Used a wood that matched perfectly. I did write to McSpadden since I had shipped the dulcimer once to them. They were willing to pay for the repair since I made the effort to ship it to them first under warranty. This restores my faith in McSpadden. Just wanted to let everyone know the update. 

Thanks for asking. It is still under warranty, so I shipped to McSpadden. They heard nothing and replaced the zero fret. However, the problem was still there. Then I let my teacher hear and he clearly heard it. So, my husband did some fret adjustments. He put thin paper under nut slot of bass string. It is still there, but not as bad. I found another place willing to look at it. Just finding the time to do the drive. When I finally get fixed, I will definitely let you know. The bass string was sitting on the 3rd fret. The nut needed adjusting but is unmovable. My husband figured it out. But he is a guitar teacher and not a luthier so he adjusted a fret just a little. However, what he did made it bearable to play. 

@silverstrings , did you ever figure out what was going on with this?

Yes, I did buff both nut and bridge. Maybe nut needs it more? Yep, hard to find .028 loop end strings. I thought about thinner. I didn’t know if that would make it worse. I tried 3 bass strings. Two of them were phosphor bronze and other steel? The latter was better. Thanks so much for getting back with me.

With a 26" scale, you could 'try' out a .028 wound bass string tuning it to low D.  But I definitely wouldn't try thicker than that or it might snap.  Your 26 gauge current bass string is already within the high range for thickness.  Give it a try- the worst that can happen is it would break the string. Make sure you have spare .026 and/or .024 wound bass strings to put on if the .026 breaks.   ;)

Because the .026 is already on the heavy side, you could actually test out a thinner bass string- like a .024 or even a .022 wound.  It sounds counter intuitive but remember the buzz started after changing strings. If you have spare strings it doesn't hurt to try this stuff out... strings are cheap.

p.s. did you buff the nut slot with the waxed floss as well?

If the buzz still occurred with the string sitting on top of the bridge without being in the slot, that suggests the string action being too low is not the culprit.

Strumelia, any suggestions on strings to put on and see if there is still a vibrating buzz? Thanks

Dusty Turtle, I looked and neither McSpadden have the v shape in the slots! Weird, but my Black Mountain Dulcimer does have the v shaped slots. Which no matter what I do on it, no buzz. That is why I don’t think it is me. My all Walnut McSpadden does not buzz either. Just my FAVORITE sounding dulcimer. I believe there has to be an answer. 

Yes, Salt Springs, I did tighten up everything on the headstock. I appreciate you input.

Yup, check the hex nut directly under the string too if you have not........my McSpadden is a bit touchy and I can make it buzz if I use pick and hit the string too low.  Good luck with it all!

Update:  I just checked mine and two of those hex nuts were a bit loose...you could actually turn one with just two fingers. 

Wow, I appreciate all the responses!

Strumelia, I am tuning to DAD.

Yes Salt Springs, that was my thought about tightening the 2 screws on the tuning peg. I did that a couple of days ago.

Dusty Turtle, I will take a look at the nut slot. Since I have shipped once already, I may have to have someone else look at it.

The bridge is unmovable so I can’t move it. I did however try to see if buzz sound happened when I put on bridge without being in the slot and it still buzzed. 

JUst as an experiment, check the nut below the bass string tuning peg and see if it is tight as compared to the others.  I have a walnut/redwood McSpadden and for whatever reason the bass string peg will loosen itself there as well as the knob on it.  Perhaps the buzz is in the peg since it seems that you have tried just about every string adjustment you can think of.  Likewise check the screw underneath the peg and see if it could be loose.

Something I learned recently is that the slot in the nut should not be flat. It should come to a little point, so the string sits in a single point rather than resting on a level surface.  In other words, the slot looks like a V if you look at the nut from the string's point of view, but if you look at it along the nut itself, the slot inside should look like a ^ rather than be flat.  If a string sits along a flat ledge inside the nut, it can move around and buzz.

I have a nice small dulcimer that had a buzz on only one string and it was driving me crazy, but as soon as he heard it, the repair guy at a music store diagnosed the problem. He took about ten minutes filing the slot to increase the point, and the problem was solved.

I'd be surprised, though, if McSpadden made that mistake in the original build.

And I should also say that I sometimes get a buzz on strings and I'll just change the strings and the buzz goes away. I can't explain why older strings would buzz, but it's such a simple fix I've never worried about it.

Your string gauges seem reasonable to me.  I use 26", 16" and 13" on my 25" VSL dulcimer tuned DAd.

and what notes are you tuning to?

You could try flipping the bridge over to eliminate the bridge [notch] as the problem. Hopefully the slant of the string will counter the bottom being square. 

I have a 26” VSL. Melody String .012 , Middle String .016 and Bass .026

Yes that might help. Try the next larger gauge just for that string.  A heavier string and a lighter string, both tuned to the same note: the heavier one will feel tighter and be at higher tension. Dont go too very thick or the string might be at too high a tension and break.  Just like for example from a .022 to a .024 wound.

There's also a very remote possibility that it's actually a different string causing the issue, but that the sound manifests itself through the bass string vibrations. In that case, changing all strings might help.

What's your VSL/scale length from nut to bridge, and what gauge strings do you have on now, and what notes are you tuning to?

Strumelia, do you think maybe putting a little bigger gauge would help? My husband suggested that might help. I put the capo on and wow, there goes the buzz. They don’t have any redwood for a while and other than this minor issue, I really don’t want to replace. 

Okay, will do. I think I will not saw at the bridge anymore. Like you said, I don’t want to overdo it. Thanks so much. 

That's great that you're maybe improving things a little.  Be careful to keep the flossing gentle- you don't want to deepen or damage the slots by 'sawing'. Do both the nut and the bridge slot, since the vibration can originate from either end yet still wind up buzzing at the first couple of frets near the peghead end.

Hmmm, I am making sure about my technique and your article is GREAT in describing finger positioning. For the most part, I learned the correct way but do get lazy. That does help. Weird, that when I am lazy the other two dulcimers (another McSpadden and a Black Mountain) are not SO sensitive. Jim did recut the bridge slots and the nut looks good. However, it is still different than the other McSpadden so I did the floss. Both technique and flossing it, tee hee, helped some. Still kind of there from time to time. 

This dulcimer has such a great “sound” that I really don’t want to replace it. That is why I shipped it in the first place. I am sure there is an answer. I really appreciate your input.

I will be interested to find out what Jim Woods says. I do respect his opinion. 

I may floss some more. That makes total sense. That problem was not there UNTIL I changed the strings.

More thoughts... you say the buzz was more pronounced when you had 'flatwound' guitar strings on, and the buzz was less so when you switched to regular wound strings.  To me this may suggest that the buzz occurs when the string is not well seated in either the nut slot or the bridge slot.  If you have a slot that is shaped with a "v" shape nut file, and the string is heavy and is round, then it leaves a very tiny space gap at the bottom trench of the V.  This often causes buzzes. It's a little like putting a round peg into a square hole... you get gaps.

Talk to your husband about this.  I have sometimes solved this 'slot gap' string seating buzz problem by doubling or tripling some waxed dental floss and gently but firmly running an 8 inch length of that that back and forth in both the nut slot and the bridge slot.  (don't saw it too hard, you don't want to file down the slot) That can buff and smooth any microscopic burred edges in the slot and helps the wound string seat more snugly without actually filing the slot.  Obviously you loosen the string first and pull it over to the side while doing this.  It's an easy quick trick to try that won't cause harm to anything, and may make a difference.  The fact that a flatwound string buzzes more than the round wound string makes me suggest this.

Have you looked at this blog post I wrote about finger position when fretting?  Hand/finger position can often cause buzzes and other weird sounds.  I've seen this issue many times in players...

https://dulcimer-noter-drone.blogspot.com/2012/07/lazy-fingers.html

Have you had your husband play on the string while you listen for the buzz?  If it only happens when you are playing, that points towards something you are doing. Maybe there's nothing actually 'wrong' with this dulcimer but it is just a bit more sensitive to your personal playing methods/habits in some way.

ALSO: does the buzz happen on the OPEN bass string?  Does it happen only on certain notes fretted, and not when you play the string open?

Skip, Yes, I have tried the nut/bridge and the Luthier checked the action. I sent a video to him. Hopefully, that will answer some questions.

Have you checked the action, nickel/dime method? A strip of paper under the nut/bridge will raise the action.

Edit: In your user photo it looks like you're fingerpicking. From your hand position it looks like you may be slightly lifting the base string with your middle finger. 

I retuned the dulcimer to EBE and that did not do the trick. Jim Woods is great. He is willing to replace, but wants to make sure that it is not something I am doing. I understand his point. If he makes another dulcimer for me and it is still doing it, then, it is me!!!  I love the sound of this dulcimer. I would rather it be, “pilot error” as Ken Holme stated.  What is weird is that buzz does not happen on my other McSpadden (which has a 1-1/2 fret) or my Black Mountain (which is an inferior dulcimer). I will let you all know what we find out. 

As a test, try tuning just that bass string up by one step- if it is in D tune it to E.... and see if the buzzing stops. If it stops, then you'll know that the bass string is too light a gauge- switch it to the next thicker gauge and that will make sure it won't be tickling the frets when vibrating.

If the test does NOT eliminate the buzz, then the problem is not likely with the string, but could be with the nut/bridge slots, or the frets, or your fingering techniques.  Since Jim Woods has checked out the instrument thoroughly, and also since he cannot hear the buzz happen for him, then perhaps your physical playing technique as the most likely suspect. 
Can you hear the buzz if you get your husband to play the dulcimer or strum it?

I had the same issue with one of mine many moons ago.  It just about drove me nuts because for awhile everything was fine and then the buzz thing would crank up.  I finally figured it out when like Ken points out, it was me catching the string  on the strum and pulling it up a bit.  I also put a bit of a heavier string on the bass then I found was included in the string sets you buy.  As I recall I went from a 24 or so to a 28.  (I think the Strother string calculator is a bit light.)   That helped a great deal but it still happened.  Steve Siefert once mentioned that a heavier string could correct some problems like this when using a 155 tuning.

The next time it happened I froze on the chord and it dawned on me that my bass string was not being held with the same amount of pressure as the others allowing it to buzz......

I am not saying that your issue is the same, but in my case it was due to several different things.......too light a string, a weak ring finger and an inconsistent pick angle.  I also switched to a lighter gauge pick too.  That made it harder to lift the bass string.

As suggestion, if you have not tried this, tune it up a to maybe EBE and see if the higher tension makes it stop.

All in all, Ken's comment fit my predicament and was right on the money.

This is absolutely the most difficult problem a player and a builder can have.  Frustrating is too mild a word!!

A sometimes buzz can sometimes be traced to "pilot error". 

I had a similar incident to investigate recently for another luthier friend.  A client bought a dulcimer and loved it but had issues with random buzzings.  The instrument was shipped back and forth a couple times halfway across the country with no satisfaction.  The luthier contacted me because I live relatively close to his client, and asked that I basic do a complete checkout and re-setup the dulcimer.  I did so, with the owner watching like a hawk.  Then when she would try it, yes, once in awhile there was a random buzz.  I kept watching her play, and realized that she would get those  sounds because she was not consistent in how she picked the strings.  Occasionally she was picking up, not across, and it was just enough to unseat a string in. its notch, which would buzz a split second later as it was plucked again and returned to its proper place in the slot

I have s 26” VSL McSpadden in Redwood/Black Walnut. Only had a few months and when my husband changed the strings (he is a guitar teacher so knows what is doing), the bass string started buzzing sometimes at the headstock. It can happen on open chords, 1st fret and 2 fret. We changed the bass strings to squeakless then tried regular, which was not as bad. I shipped it and all frets they checked were good. They did replace zero fret and did some work on middle string. This dulcimer has great tone and I hate to return to get another one built. However, Jim Woods and several luthiers who work there could not hear the vibrating buzz that I was talking about. He is a great guy and trying to help me with this issue.  I thought about “living with it”. But I hear it a lot and it is annoying sounding. It did not sound that way before we changed the strings. I tried paper under bridge area, nut area, tightening screws, etc...