Odd. 

I know that in organ building a similar effect is actually sought out: when a 16' D and 10 2/3' D (sounding an A) are properly tuned and played together, a resultant pitch, an octave lower, a 32' D is heard. In organs this is handy because not many buildings can house a veritable forest of huge pipes that a proper 32' rank would require.

Kind of odd that it should be the fundamental A you're hearing and not the D.  Kind of a neat effect, though!  Does it only happen with that combination? Or do C/G, E/B and F/C produce similar effects? What happens when you slightly untune one of the strings just a bit?

Will do, and I'll report back....

Try putting a tiny piece of toilet paper (a couple of thicknesses) under the string where it crosses the nut

Though my knowledge is very limited, I'm thinking you do not have a wolf tone; at least, on a cello, a wolf tone presents every place that note is found on the fingerboard.  As you note, you are getting a sympathetic vibration.  Please let us know how all goes?  And good luck! 

Not yet.  I'm going to have her change both the bass and middle strings, since I think they are original to the instrument.  After that, we'll see if that gets her back to where she was this time last week.  If that doesn't solve the problem, I'm going to wonder what has come loose and started vibrating sympathetically with the middle string.  "Damping" the middle string does stop the wolf tone....

@spineloccio Any change in the unwanted tones your wife's McSpadden was giving?  

Wolf tones like that can be notoriously hard to cure.  However.  This is probably something that has crept in from having the instrument used -- tuned, re-tuned, changing environments, etc.  Start by slacking way off and then re-tightening the middle drone string, making sure that you have a snug fit in the notches at the nut.

FWIW, since strings can be tuned to any of 8 notes in an octave, we generally refer to them by their "course name", not the note they are tuned to.  The dulcimer has a Melody course, the middle drone course, and the bass course.  Each course can be 1 or 2 strings.  

My wife has a McSpadden that is about four years old.  The other day as she was practicing she started noticing what she thought was a buzzing string when she played the low D on the second fret.  I started looking at it this afternoon, and there is no buzz when the string is played by itself.  Instead, when the low D is played and the A is also played, there is an octave undertone on A.  It just happens to be very obvious when the D is played on the second fret.  Any ideas, please?