Is it for sure the seat and not the plastic right behind or over your shoulder? If you have the seat back enough to where it is touching the bulkhead it can sometimes squeak as the plastic and leather rub together. Readjusting so there is a sliver of space between can help, or even leaning it back harder to keep it solid. The other problem is where the plastic meets around the little side window, that can get really annoying on bumps and such as every little thing will make it creek and pop.