Yep -- a small triangular file belongs in every dulcimer players kit.  Good catch Dusty!

Great insight Dusty. In fact, I have a banjo with a string that rings out in an annoying way- I will try this fix next time i take it out to play! I have a little nut file that has served me well for years.

Sorry about the confusion everyone. Someone has to be the most clueless member.  Dusty, just this morning I noticed the mid string was sitting in a very shallow slot at the bridge. I could feel much more vibration when i put my finger on it than the others. I deepened the slot and it does seemed to have helped. The vibration at the bridge is gone. I think you're right. I'm going to consider the problem solved. Thanks for all the input everyone.

@gennaro, before we can really help you, we need to figure out what exactly is up with that middle string.  If it's not out of tune, and it's not buzzing on a fret, what could it be?

I have one idea. I had a dulcimer that had a minor problem with one string. It would kind of ring out louder than the others with a very faint buzz.  The problem was not the frets, but something else a repair guy at a guitar shop diagnosed right away: the slot in the bridge and/or nut was too flat, allowing the string to vibrate.

Basically, as the string goes through the groove or slot in the bridge and nut, you want it to sit on a single point in the wood or bone or whatever the thing is made of.  Is that slot is too flat, it allows the string to vibrate and kind of ring out louder than the other strings and make a noise that is not exactly a buzz, but something less obvious but still annoying. This happens more often when the nut and/bridge are made of wood rather than a harder substance like bone.

The guy who fixed my dulcimer that had this problem did so in about 5 minutes.  If you have a really small file to get into those grooves, you might just run it at an angle on either side of that groove in the nut and bridge, trying to create a single point on which the string will sit.  If you don't have such a tool, any guitar shop should be able to do this.

I don't know for sure that this is your problem, but the difficulty you have describing it sounds really familiar, and this is my best guess.

OK if the scale length is 27" then forget everything I said.

It could be that the MD is particularly resonant to B, which makes that note 'stand out' more. Since you are trying to play the tune in Aeolion [capo at 1] mode it may work to try tuning to DAc.

After checking the link, I see I may have been confused on the scale length. The nut to bridge is 27 1/2. The actual fret board is 22 1/2. 

Thanks for the advice and the link. I'll go heavier on the strings. As far as filing the frets, I'm always tinkering. It's a no name dulcimer I bought from a friend. I plugged it into an amp on a whim and it actually sounded better. 

Ok well that helps a lot. 22 1/2 " is a very short scale length... practically like an epinette.

You don't say what gauge your other strings are - that would help!

But .012 for a middle string at 22-23" scale length is too thin a string for DAd or CGC... and whatever tuning you use, a string of .012 for a middle string will be rather loose... so that when a capo presses the strings even further down than the nut slot level, that middle string is likely to vibrate against frets when strummed open. It's a shame you already filed down frets, as that's a more drastic and risky solution (sort of like cutting off a table leg if the table wobbles).

All you need do here is put on strings that are more appropriate to your short scale length. A heavier set will not be lax and vibrate so much that they create sympathetic ringing against the frets. The middle string will likely need to be at least a .014, for example. How did the current set of strings get chosen?

The Strothers gauge calculator is very helpful when ordering strings: http://www.strothers.com/string_choice.html

FWIW, your described 'ringing' is not a string that is 'dischordant' (meaning a note out of tune)... it's more a noise, a wolf tone, or a buzz type issue.

Scales are 22 1/2 inches. The middle string is 12 gauge. When I play, or attempt to play the song in DAD the middle string doesn't annoy me. However when applying the capo to the first fret as advised by Mr Gilston, the middle string is tuned to B which dominates the song when strumming it opened. I would discribe it as ringing, and to my ear, annoying. There are a few others playing this on Youtube. Guess I could ask how theirs are tuned. 

Also- please elaborate on what you mean about the middle string that it sounds "off" or "dischordant:
Do you mean it's actually out of tune, meaning not the right note, sharp or flat?  OR
Do you mean it has a bad tone- like a buzzy sound or thuddy/twangy. Need to know this!

What is your VSL/scale length from nut to bridge? And what gauges are the strings do you have on this dulcimer? These factors are important in figuring out your problem.

(note: don't attempt any tuning higher than EBE or you may break strings)

EBE definitely wasn't the way to go, but I recalled seeing a post that mentioned CGC as being compatible with DAD songs. I tried it and it seems muted or softer, which tames that middle string. It may take a bit getting used to as the strings are not as taut. Thanks again for the input.

That way, if things are still discordant, then the issue is definately not the capo itself causing problems.

Capo is on first fret. I had that setup, as it's the one Mark Gilston recommends in his book that i purchased. He plays it in a Youtube vid, and of course it sounds great. I'll try tuning to EBE. I think that's it. I'm very musically disinclined. I basically just learn  songs by Youtube vids and making tabs from them. Thanks.

You're playing "King..." from DAd tuning -- capoed where?    And it's when you're capoed that the open middle string is discordant?  

Have you considered re-tuning from DAd to whatever the capoed tuning is, or to any other tuning?

Even though I know it's beyond me, I've been trying to learn King of the Fairies for a few months now. I almost have it but when I strum with an open middle string It sounds off to the point, I'm considering ditching the song.  It's not a problem without the capo while playng other songs in DAD. 

Fixes I've tried. I noticed with the capo, the strings were almost laying on the second and third frets. I laid a straight edge on the first five frets and sure enough, the second and third were high so I filed them down. I then put new strings on. I used a 12 for the middle, which is what I had. It's a little better, but that middle string still dominates. Any insights appreciated.