If your house is sitting on stumps, chances are you will have some sort of particleboard base which acts as the main floor support. It is what your flooring surface is resting on and this is called a subfloor. Over time, as plywood subfloors that sits under carpet expands and contracts, the nails can come loose and the subfloor separated from the floor joists. Although it's not a safety concern, it's an annoyance: squeaky floors under your carpet.
This Old House's Tom Silva demonstrates in this video how to fix those squeaks using a special adaptor and screws from O'Berry Enterprises' Squeeeeeek No More kit. It costs about $44 on Ubuy Australia. You can re-fasten the plywood to the floor joists without having to pull up the carpet. The screws pass through the carpet and their screw heads snap off so you won't see or feel them after you're done.
It takes about an hour or two for an average-size room, and you'll need a cordless drill, hammer, tape measure, and nylon string.
Check out the video or the post below for the detailed instructions.
How to Quiet Squeaky Carpeted Floors [This Old House]