Those little annoying floor squeaks, common in many homes, can occur after the house has settled and flooring lumber has dried out and shrunk. When you walk across the floor, the boards rub on nail shafts or against each other. It's this rubbing that produces the creaking and squeaking noises. Squeaking also comes from loose sub-flooring whether it’s OSB (particle board) or plywood.
Squeaks are also caused by changes in temperature and humidity which cause the expansion and contraction of wood. If the wood expands enough it can actually pull the nails slightly. The sub-flooring and joists can warp causing a separation between the sub-floor and joist or between the finish wood floor and the sub-floor. The wood moving up and down on the pulled nail causes the squeak.
Repairs from below:
If the floor is over a basement or crawl space, you can go below to make the repairs. Have someone walk across the floor in the area of the squeak while you listen from below. When you hear a squeak, have the person above tap on the floor so you can pinpoint the exact spot. Next, take a thin wood shim and coat it generously with some carpenter's glue. Tap the shim into the space between the joist and sub-floor. Be careful not to drive it in too far because you will raise the flooring. You just want to fill the gap between the sub-floor and the joist to take out any "give" in the floor. For extra support, drive a 1 1/4-in. drywall screw at an angle up through the joist and shim and into the subfloor.
Squeak-Ender Hold-down bracket
Hold the Squeak-Ender's steel mounting plate against the joist, and then screw it to the plywood subfloor directly under the squeaky spot with the four screws provided in the kit. Slide the bracket onto the threaded rod, hook the bracket under the joist, fit the rod into the center hole in the mounting plate and install the included nut. Now just tighten the nut with a wrench until the squeak goes away. If the squeak ever returns, you can give the nut a few more turns.