Q: I have a 2001 GMC Sierra with the 5.3-liter engine. After I park the truck in the garage and shut it off, I hear a "pop" noise about 10 or 15 minutes later. It's a loud noise and I can hear it inside the house. It sounds like it's coming from the rear.
A: It has to be one of two things: the exhaust or the fuel tank. As the exhaust system cools and contracts, sometimes you'll get a muffler or catalytic converter with a section of metal that meets all the criteria to make noise. The metal expands smoothly and quietly as the exhaust system heats up, but on cool-down, the metal contraction hangs up at a point, until it finally pops back into place. It's a similar situation with the fuel tank, only it's made of plastic, and internal fuel tank pressure (along with heat) is causing the effect. When you park the hot truck, tank pressure is at a high point, and as it cools down, pressure decreases. That's when you get a section of the tank, which flexes back into its low-pressure stance with a bang.
The exhaust will usually produce a more metallic sound. You may have to spend a little time on your back on the garage floor, waiting for the sound to occur in order to narrow down the cause. But try this first. Next time you park, remove the gas cap. If you hear the pop as pressure is relieved, or the noise no longer occurs a short time after, that proves the tank to be at fault. Typically, the only fix is replacement of the defective part.
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