-- Last Updated: Oct-10-12 1:57 PM EST --
Check to see that connections are tight, but don't over-tighten them, as that can dent vinyl gunwales. If that isn't the problem, try taking the seat hangers apart and gooping all contact surfaces with some sort of anti-squeak material. Yes, I'm serious. On my rowboats, the seat-frame ends simply rest on wood rails, and are secured against forward-backward movement with ropes. This doesn't totally eliminate movement of the seat, and the boats flex a bit too which also causes slight motion at the seat-support points, and sometimes that leads to squeaks. If I rub the squeaky contact surfaces with paraffin, there's nary a squeak for the rest of the summer. There might be better materials to try than paraffin too.
Kayak & Canoe Outriggers
Free Standing Boat Racks
The Kayak Wing