All floors shift over time, and this movement plus the increasing dryness of the boards can cause the subfloor to come loose from the floor framing members. The drying of the wood allows the nails to come loose and as you walk across the floor the movement of wood and metal sound like a symphony. Attaching the plywood subfloor to the joist at the edges and in the field of each sheet of plywood is highly recommended. Your problem also could be the ends of the floor joists have loosened and are rubbing against the rim (perimeter) joist or the joist hangers. If this is what’s happening, you’ll have to pull up the carpet and pad, and open the floor to secure the ends of the joist to their hangers or to the rim joist – it gets costly when this has to be done. Your current direction is the best way to begin.