Have plywood under carpet that needs fastening more securely in places. Most stud finders not capable of locating studs through layer of carpeting. Any way to find studs and, if so, best way to drive fastener through to the floor.
I've seen a gadget on the TV show This Old House where you can tighten down the loose spot in the subfloor using a special screw. Once it's just below the surface of the subfloor, you break off the head of the screw. Finding the joist was a hunt and peck though.