Weirdest thing.....

I had a an intermittent(sp) groaning noise coming from what I thought might be the transmission. It only occurred after the truck warmed up with a bit of driving. A dull groaning noise, lasting about 2 seconds and happening about every 30 seconds.
Upon closer inspection, listening, it turned out to be pressure being released from the fuel cap every so often; sounds like a sick cow. Scared the heck out of me; thought it was maybe torque convertor.

Has anyone experienced this? I suppose I should replace the cap?

 

vapor pressure, check your vaccuum going to the charcoal canister. its supposed to relieve pressure from tank and also get new cap or drill hole in the exsisting one