Help r/Guitar: I have a three-string dulcimer, and one of the strings rings. How do I fix this?

Do you mean like a string buzz?

Which string? Is it buzzing because of contact with a fret or is something else buzzing, like a crack (run your ear along the string and dulcimer to see where the buzz is loudest, if you think it's a particular fret it's hitting put some thin tape on it to see if the buzz is muted to verify the string is hitting that fret). Holding the instrument in profile in front of a CRT monitor or TV and playing the string will appear to slow the up/down displacement so you can see it it's arc more clearly, to see where it's touching

Solutions:

• The slot that the string fits into in the nut is too deep. Look at it to see if that's the case, raise the slot a little bit with a shim or something that you can fit into it that will raise the string without damaging the nut - plastic, paper, card etc. to see if it fixes it. You can even test this with the strings in situ - jam a thin knife, screwdriver or any thin lever behind the string (behind where it slots into the nut and in between the nut and the pegs) and lever the string up a tiny bit - it's effectively (nearly) the the same as putting something in the nut slot to raise the string. Protect the wood with cloth so no metal comes into contact with it. It's not a permanent fix but a way to verify it's the cause of the problem - the fix would then be to replace the nut. You could raise the entire nut with a shim but if two slots are fine and one is too deep then by raising it you're raising the other two strings, unless it's one on the end in which case you can raise it at one end

• The whole nut is too low. Less likely than above but possible. Solution is to raise the nut with a shim. If it's easy to remove the nut (they're often held in place with a bit of glue but really it's the string's pressure that's holding them on so they're actually easy to get off) then this is an easy thing to do to test - just stick a pit of plastic under it, restring and test. Only glue it back on once you're certain it's fixed and the other strings haven't been raised too much.

• One particular fret could be too high (check if it's unseated so it's raised). This is very unlikely to be the cause if it's buzzing on open strings though. It'll be hitting the fret because the string's too low rather than because the fret is too high.

• Loose bridge, crack, something loose (e.g. a peg head vibrating), as Jones suggests

Obviously, once you've determined the exact problem, do some Googling for more info on the fix - e.g. don't go removing nuts, filing, gluing bridges etc. before reading up on it a fair bit. The key is to 100% know what the problem is - the fix is then easy with enough thinking about it.