Loud bong noise from gas tank

:confused: I have a 1997 suburban with a 5.7L 2 wheel drive with 185,000 miles on it. I bought it not too long ago and after I shut off the motor and the suburban sits for about 20-40 seconds I hear a loud bong noise like somone hitting a 55 gallon drum with a mallot. this also happens when I start it up and it runs for about 20-40 seconds. its the darndest thing. Im just conserned the darn thing is going to blow up on me or something does anyone else have this problem and does anyone know how to fix it. the drivability and performance of the suburban is good, I have no other problems with it:banghead:

It may be the secondary EVAP system drawing a vacuum on the tank greater than it should. Typically, the vacuum should be around 1" Hg. There should be a test port in the engine bay to check this. You might also try to loosen the fuel filler cap before you start the engine to see it the noise occurs. If there is no noise with the cap loose, but it happens after tightening the filler cap, the EVAP system should be checked. BTW - Don't drive around with a loose cap to "fix" the problem.