What would cause the floor to creak and how can I eliminate the noise?

Irritating squeaks that originate between the joists are most likely caused by the wood floor rubbing against the underlying plywood subfloor. You'll need to stop that friction by driving short screws from the underside of the subfloor into the bottom of the finished flooring.

Loose boards are usually the cause

Talc between the boards can help. If you can pin point the board, and you own the place, you can screw in back down.

Your floor joists may have dried out a little. If you can get to the underneath, place a shim between the floor and the joist. Have someone bounce on the area so the person below can identify the correct location.

if you can get to the under side and locate the area where it squeaks at. If this is your home or your landlord won’t care. You can toenail thru beams by drilling a hole a little larger then the diameter of your screw at a angle thru your beam. Either add some weight on the floor upstairs or have someone stand there and run Sheetrock screws into your floor boards to pull them down tight to your rafters. Make sure your screws are not too long that you screw all the way up thru your floor though. Those screw points hurt, especially walking on the floor barefoot! LOL

If rafters are not close to the area where it squeaks, you can take some 3/4 plywood and glue and screw the plywood underneath between the rafters to stop squeaks as well! Have done this in many older homes over the years with squeaky floors and it’s always solved the problems! Good luck