I just recently bought a used '01 Sport Trac and I've noticed a plastic burning smell when I first use the A/C. It is not the "musty" smell. It goes away after a minute and doesn't do it until the next time I use the A/C again. The dealership did tell me they replaced the drive belt and tensioner. Every once and a while the belt will let out a short little screech when I start the truck. Maybe the belt slips when the A/C first comes on, I don't know. Any suggestions?