It's happened twice so far in the 10 years I've had the car.I'll be driving along and I hear it go thud.I get home and I'm in the house and it thuds again.I get a lot of pressure on the cap after driving so for a short term cure to keep from replacing another tank I've drilled a hole in the cap. Anyone have this problem?Thanks-Mike

 

I have taken out my charcoal canister, and ran the vent line down below where the canister was. As the tank emptys, the pressure behind the cap does increase on my car, but it has never done anything like you have described. The most it has done is blown the cap into my hands as I was unscrewing it.

 

Check/replace the plastic vent valve up under the car, near the filler neck.

 

Morley's got it. It's because as your engine drinks the gas, it can't replace it with air. So it establishes vacuum and sucks the tank in. It makes the bang noise when it finds a way to vent and it releases the sides of the tank.

 

OK thanks,I've seen that and wondered if it was related.