Over time, old floors settle and develop squeaks. Although the noises might be handy for catching truant teenagers, they also can be irritating. Not every sound a floor makes can be dampened, but more egregious ones can be dealt with from either above or below the floor.
Once you’ve determined the squeak’s location, drill a pilot hole at least ½" in from the edge of the board. (Avoid drilling into the subfloor.) While holding down the board, hammer a long finish nail at a slight angle through the pilot hole and into the subfloor and joist. Countersink the nail so the hole can be concealed with wood filler. Choose a filler color that matches your floor.
Position yourself in the basement or crawl space, and have a helper walk across the floor above so you can pinpoint the squeak and look for movement. Once you’ve determined the moving spot, gently tap a shim into the gap so that it fits snugly. (Don’t drive the shim too far into the gap; it can cause the floorboard to bulge.) Draw a line on both sides of the shim to mark its depth, then remove it and coat both sides with carpenter’s glue. Tap the shim back into the gap up to the mark. Allow to dry for at least four hours. (Avoid walking on that area of the floor while it does.) Once the glue is dry, use a utility knife to trim excess material from the shim.