We just purchased a used 2002 GMC Sierra Extra Cab Short Box 1500 with 28k on it. It seems to be in good overall condition although the underneath appears to be a bit rusty for the age.

Our concern is a popping sound that has occured 3 times. This has occured once when the truck was running but in park and twice right after it shuts off. It sounds like an expansion of sheet metal after being pushed on. The low coolant light came on but other than that no other indiations of something being wrong. Filled the coolant and it still made the popping sound. Seems unrelated?

Has anyone else had this problem?