Boards that rub together cause squeaks in hardwood flooring. The offending boards may be in the subflooring, in the cross braces under the subflooring or in the hardwood itself. Often the hardest part of fixing the squeak is determining its source, according to the New York Times.
Subflooring is the material that provides support for the hardwood. In older homes, the subflooring may be dimensional lumber nailed to the floor joist at an angle to the direction of the finish flooring. Newer homes use plywood or oriented strand board for subflooring. In addition to squeaks caused by rubbing joints, some subflooring noise is due to a board rubbing on a slightly loose nail.
Underneath the subflooring and between the floor joists, builders install braces in an "X" pattern. They design these braces to prevent the support timbers from twisting. Ideally, one side of the "X" is slightly above the second half to prevent noise. However, sometimes the installer makes a mistake and the boards touch. These boards rub together to make the noise.
Improperly nailed hardwood flooring also makes noise. It is sometimes possible to eliminate hardwood squeaks by pouring talcum powder or graphite powder over the squeaking board, covering it with a towel and stepping on the towel to work the lubricant between the boards.