I have a 1992 Chevrolet 2500, 6.2 turbo diesel. When I start the vehicle there is a loud popping noise on the passenger side in the cab. I believe it to be something in the heater/AC system like a door trying to open or close. It usually stops after about 1 minute. Where do I start to look to get this fixed? And how do I get to the problem if it is inside the heater system?