Fret buzz present only at specific situations - how to explain?

Question

I hear a strange fret buzz on my classical guitar, but only at a very specific situation. If I do both of the following, I hear the buzz:

1. Hold 5th string at 8th fret (note F), but not play it
2. Pluck the 3rd string on 9th fret (note E)

In this situation, the 5th string buzzes. The buzz is not very annoying, but is consistent (happens all the time). I did the following experiments with it:

1. Tried another guitar - the buzz doesn't happen
2. Played the E on the first string - the buzz becomes very weak, almost disappears
3. Played other notes on the third string - the buzz doesn't happen
4. Held other positions on the 5th string - the buzz doesn't happen
5. Tried various orientations of the guitar - the buzz disappears in a few situations (e.g. the guitar is flat on a soft surface)
6. Tuned the 3rd string up/down a little - the buzz disappears
7. Tuned the 5th string up/down a little - the buzz disappears

So I have the following questions about it:

• Does the 5th string graze the 9th fret, or can it be any other fret?
• How can one explain this physically? Can I say anything more specific than "there is a spooky resonance somewhere"?
• Is there a good chance I can get rid of this by filing down the problematic fret?

Answer 1

WIth that fingering, have you tried plucking the 5th string on the nut side of your fretted point? You will find that the vibrating string between the nut and 7th fret gives the same note you are plucking on the 3rd string.

Normally the nut side of each string will resonate a little if damped (which is why for tapping passages, many musicians use a nut damper like a cloth or similar), but it sounds like your 7th fret is perhaps a little worn down so the string is buzzing against one if the lower frets.

Definitely do not try filing down a fret to try and fix the problem. If you haven't done it before you will make it worse. Take it to your local guitar shop and get them to sort the frets out.