If you can get to the back of the stairs, by opening the ceiling underneath , shot adhesive caulk or construction adhesive at all joints of the stair under carriage. Once hardened any movement of the stair members should stop when stepped upon elemanating squeaking. If you cannot open the rear then spread the carpet pile and pre-drill the stair at the locations that squeak. Then install trim head screws into these areas. Make sure that you pre-drill with a bit that allows the treaded part of the screw to slip through the first piece of wood easily and a smaller bit to finish the job into the second piece of wood below so the wood pulls in tight. Experiment with trim head screws and simulated wood strips before yoou drill the steps. Your goal is to suck the wood steps tight to the under carriage to elimate movement when stepped upon. Good luck. any other questions you can reach me at Nick@plasteringbynicholas.com
Regards,
Nick
