Squeaks are often caused by floorboards rubbing against each other or from sub-flooring loosening from the floor joists.
If you can access the underside of the flooring examine the joists. If you can see a gap between the joist and the sub-floor that could be the squeak.
Use construction adhesive to silence the squeak. But first pull on some gloves, - this stuff is sticky and messy.
Run a bead of adhesive along the joist, then use a disposable spoon or a paint stick to push the stuff into the gap along the joist.
You'll want to use something to smooth the bead that you can throw away because this adhesive is nearly impossible to remove.
Repeat the process for the other side of the joist, then any additional joists.
Allow the adhesive to cure and you're all set - squeaks silenced!
I'm Mrs. Fixit and it's just that simple!