I have red oak T& G hardwood floors. It is on a crawlspace with 3/4 plywood in Alabama. In the winter it has several squeaks which I have tried to fix with counter sinking finishing nails. Also alot of the boards make a swish sound like there is a small sound coming from the underlay ( if there is any). I have tried many different options. Currently have a humidifier running and this has not helped much.
Seems alot worse in the winter months.
Is there anything that can be done. It is a 6 year old house and didn't have any problems the first 3 years.

Typically in the winter months there is less humidity in the air and red oak will react to that by drawing up or shrinking. When that occurs squeaks are likely and expected. As for the sounds if you think there is a chance that it may not be nailed properly purchase some small magnets from a woodworking supply store generally the are called true earth magnets. Slide them along the joints of the boards they will be attracted by the nails or lack there of. Mark them with a piece of painters tape. They should be within 2" of the end of the board and every 8" there after depending on the length.