Andrew Spence
The old flooring is likely to be nailed down and over time these start to come loose, you should try screwing down the existing flooring into the joists, go next to the existing nails, be careful of any pipework. For most floors this will work the screws will need to be at 3 times the thickness of the flooring and you need to go in to the joist, once you have done whole floor if still squeaking in places then put extra screws in to pull flooring to joist. You will need a lot of screws to do this . Good luck
