Sound board rattle

Q: I've got a Yamaha guitar I'm working on, it has a good bass tone and is pretty loud. Only problem I've got a rattle somewhere in the soundboard. I've checked the braces with a mirror and none seem to be loose. What's the problem, Help!!
Joey

A: Rattles are either easy to find or impossible to find. There is no middle ground. To look for loose braces press on the top and watch for movement inside the guitar (with a mirror and flashlight). Try having someone else play the guitar and make it rattle. Then start poking at it. When you push on the top above a rattling brace, or loose bout, the noise will stop. Poke at the pickguard, at the tuners, at the bridge, etc.

The first place to look for rattles is on the strings.  A nut slot worn, or filed flat on the bottom of the groove, may buzz or damp out. A string might buzz against the top of the bridge behind a too low saddle. If the string slots or pin holes don't fit right the string ball can buzz. Of course, check for strings rattling against the frets. Frets loose in their slots can rattle. Truss rod nuts can rattle. Truss rods can rattle. Are we talking about an acoustic/electric? There is a lot of hardware in those that can come loose.

Sometimes you have to think "outside of the box." Are your shirt buttons or pen rattling against the back of the guitar? I spent half an hour trying to find a buzz in a stand up bass. I finally figured out that my metal tool chest was buzzing, vibrating sympathetically with the bass. I moved to another room and the buzz went away.

Now, the true horror. Some buzzes are "wolf tones," frequency spikes resulting from sound waves colliding on the resonating plates. All top quality guitars display this phenomenon to some extent. It can be anywhere from unnoticeable to unbearable. Cheaper guitars don't have this problem because their less athletic tops are incapable of producing such conflicting waves. It would be a rare Yamaha that could produce a wolf tone, but it could happen. If all else fails you can call your buzz a wolf tone.  To fix a wolf tone you must change the acoustic properties of the guitar.  The quickest and cheapest thing to try is to change the saddle.  I have beaten wolf tones in Taylors by going from Tusc to bone.  In Martins Iâve gone from bone to Tusc.  You must do something to change the flow of vibrations enough that the wave crests coincide differently.

Water from a roof leak can run down a beam and make a puddle many feet away from directly under the hole.  In a guitar some parts just transmit vibrations and some transfer them to the surrounding air as sound.  It is possible for one part to buzz, and the sound of the buzz to be projected by another part.  I use a stethoscope to focus my hearing to a small area to help in the search. I do not currently have an industrial strobe light in my arsenal, but if you have access to one you can aim it at your guitar and adjust the flash rate to stop or slow down the vibrations of the top. This is fascinating if not helpful. Good luck.