To fix a squeaky floor under carpet, locate the floor joists with a stud finder, then drive in a trim head screw through the carpet, pad and subfloor, and right into the floor joist. Be sure the top inch of the screw doesn't have threads or the subfloor won't suck down tight to the joist. We like trim screws because screws with larger heads pull down and pucker the carpet. If that happens, back out the screw and drive it back down. Keep adding screws until the squeak stops.