To get to the bottom of a pesky buzz you have to use the process of elimination. Try to find where it's coming from. Is it there when the string is played open? Does it only show up when you fret certain notes? All of the strings, or just one? Try to pin down the exact location of the buzz because sometimes this alone will show you where the problem lies. Let's take a look at these steps, one by one.
Where's the buzz?
If the buzz is present when an open string is played and goes away when you fret a note: check the nut. The slot the string sits in is probably cut too deep. The string is then too close to the first fret, and rattles on it when played.
If the first notes played on any string are clear, then you get to a fret that buzzes: you need to see if each fret after that one buzzes as you progress up the neck. If each fret buzzes, you have a neck angle problem. If only that one fret buzzes, it may be a high fret.
If the buzz happens on all the strings on any fret: look at the bridge. There are some differences between electric and acoustic guitars, but checking the action is the next step. It may be too low. On an acoustic guitar look for problems like a saddle that is cut so low that there is no longer any "break angle" of the string over the saddle. This usually happens on an older guitar that is in need of a neck re-set. In an attempt to get the action lower, the owner has cut the saddle down to nothing and the break angle is gone. This keeps the string from bearing down sufficiently on the saddle and is letting the string just rattle on it. This could cause all of the strings to buzz and the guitar to have a poor sound. On electric guitars you should make sure the strings are seated solidly on the bridge piece. Check both the bridge and all the rest of your guitar for loose parts. Experienced repair people have found the darndest things rattling and buzzing so keep in mind that sometimes the buzz can be anything. Remember that we're talking about string buzzes, and not electronic buzzes. If you have an electric guitar that is making a strange buzz or rattle unplug or turn the guitar off and make sure that what you are hearing is really from the strings and frets, and not an electronic noise of some kind. That's a different kind of buzz!
After you've found it.
If your nut has been cut too deeply: you have to either make a new nut (the best fix) or refill the slot with a mixture of baking soda or bone dust and CA/super/Krazy glue, then re-cut the slot.
If the nut slot has been cut too wide or is "sloppy": that will allow the string to move around and buzz in the slot. The nut slot should be angled back toward the peghead with the highest point of the slot at the front of the nut (toward the bridge). The nut slot should be cut with a rounded bottom, preferably with a nut file that matches the diameter of the string. The string should be able to move freely through the slot as you tune the guitar, but it shouldn't be loose in the slot.
To check for high frets and the neck's straightness: you will need a good straightedge. A long one to "read" the whole neck and a short one to span any three frets is a good start.
To check for a high fret: use the short straightedge and set it across three frets. If it rocks at all, then you have a high spot. Check up and down the entire neck this way. You can use a fret crowning file to lower a high fret but be very careful and make sure you've diagnosed the problem correctly because if you file down a fret that was not really too high, or if you file down a high fret too far, you will be creating a buzz on the next fret. You just made a fret too low and now you will need to dress the WHOLE fretboard!
There are times when a fret dress is your best choice: if you have an older guitar with worn frets that are buzzing here and there all over the fingerboard and there are little "notches" worn in the frets from the strings, you need to dress the frets.
In the most extreme cases you may need to have a complete re-fret: if your frets are grooved too deeply for a fret dress, it's time to replace them. If the fingerboard or neck is badly warped you will have to re-surface it and the frets must be removed for that job. A handplane is usually used to level the surface of the fretboard, then the radius is sanded back in. When you have the neck dead straight again you can install new frets. Fretting can be a pleasure if you know how, but it can be very difficult and discouraging if you do something wrong. If you don't have experience then read up on the subject first, and practice on a few instruments that don't matter before tackling your favorite instrument. Otherwise, hire a professional repair technician.
You now have to check the entire necks' relief: this is the small amount of bow that is intentionally left in the neck to allow the fingerboard to "fall away" from the string as it is fretted. You have to learn about relief and how much of it you want or need in your neck. It's a matter of playing style and string gauge and scale length. Some players prefer the neck dead flat with no relief at all. To get a quick idea of how much relief you have in your neck, hold down a string at the first fret, and with your other hand hold it down at one of the highest frets. If your string is lying right down on the frets between your two hands then you have NO relief. Your trussrod could be too tight and the neck bent in too severe a backbow and this can cause buzzing - it'll be worse at the nut end of the neck. If the string is above the frets by a very small amount, that is about right. A easy way to get an idea of the right amount of relief is to slide a dollar bill under the strings. It should just freely pass through, touching the string and the fret. Remember, this is just to get you in the "ball park." If the string seems high above the frets then your neck has too much bow and you'll have to adjust your trussrod to straighten it.
You can adjust your own trussrod, but here are a couple of things to remember about it:
Make sure that you use the proper tool. Trussrods have all kinds of different types of adjusting nuts so make sure you have the right tool in hand and don't use a pair of pliers or anything like that!
Don't force it! If the rod isn't easily tightened then take the nut OFF and put a drop of oil on the threads. Try it again, while taking periodic looks at the neck to see when it is as tight as you need. It's easy to break a trussrod, and hard to replace it. The rod is meant to set the neck's relief and not to adjust the action. Many times people adjust the rods too tight when they really should be adjusting other areas of the instrument.
To dress your frets: you will need to adjust your neck as straight as you can get it. Then it's a matter of levelling the frets with some type of flat, straight abrasive surface. There are as many methods used as there are repair people! Some use a good file, others use glass with sandpaper attached to it, and some use a good woodworkers' level with sandpaper glued to it. To simply describe the process, you use this tool to sand down the tops of the frets, taking only the minumum amount needed to make ALL of the fret tops the same hieght. Then the frets are re-crowned using the fret crowning file and polished to a mirror-smooth surface to make playing and note-bending an absolute joy!
Fret-dressing can be a pretty involved process, so read up on all that you can before you jump in. Get the right tools and maybe even practice on a cheapie or two before you try it on your Les Paul.
This miniFAQ was written by Mark Swanson, with additions and corrections suggested by the rest of the MIMForum membership.
There's a lot more information on all of these techniques in the MIMForum Library. We invite you to register as a MIMForum member, which will give you access to our extensive Library of archived discussions.