Next you want to cut a 1x board to fit between the joists as a straightedge and check the squeak area to see if the subfloor is sagging or arching upward. To fix a subfloor sag, cut a tightfitting piece of solid bridging and drive it up into place between the two joists, forcing the subfloor upward. Nail through the joists into the ends of the bridging. If the subfloor is arching up, secure a piece of bridging in place. Then drive a glue-coated shim or shingle into the gap between the top of the bridging and the subfloor.
Humidity can cause hardwood floors to bulge up away from the subfloor.
To fix this problem drill holes from below and drive wood screws up through the subfloor to pull the finished floor down. Make sure you don't go all the way through the floor. Also it is best to check that the tips of the screws aren't sticking through the floor. Fit the screws with washers so that they won't be pulled up through the subflooring. If you cannot fix a squeak from below you can nail from above by driving ring-shank flooring nails into the surface of loose boards.