Sprinkle baby powder, baking soda or powdered graphite over the squeaky floorboard and work it into the seams. This will lubricate the wood and should keep the floorboards from rubbing together and squeaking.
If that doesn’t stop the noise, locate the squeaky floorboard from below and look for movement as someone walks on the floor above. If there’s a gap between the joist and the subfloor, add pieces of wood shim until it feels snug.
To fix bulging or loose boards, tighten the boards with screws inserted from below. Use wood screws whose length will penetrate the subfloor but stop ¼-inch below the surface of the finished floor.