Loud "Pop sound in rear 10 seconds after car is off

[MidnightSpecial]

My uncle has been a GM mechanic for years and has lots of experience with the 3.8 and this one even has him puzzled. My cousin has a 1998 Grand Prix GT coupe with like 100,000-150,000 (unsure). She says about 10 seconds after she shuts the car off there is a single, loud Pop, or Clunk noise coming from the back somewhere. My uncles only ideas are either the Gas tank is contracting and expanding for some reason or there might be a baffle loose in one of the mufflers. I'm thinking the gas tank. I forgot if they are metal or plastic. Would it be possible for the fuel pump to cause a vaccum in the tank (popping in a flat spot on it) because the vent hose is partially plugged? Then when the fuel pump turns off Flat spot or inward dent pops back out? I don't know? Anyone else experience anything like this?

Thanks,
MidnightSpecial

[skidoojames151]

there is an easy way to see if its the gas tank...try taking the cap off and see if it makes the sound

[BillBoost37]

Most likely it's expansion of either a muffler or such. My truck does it...like clockwork. I've checked, it's muffy. I made Muffy a deal, she doesn't bother me, I don't bother her.