The last two time i put in gas i herd this air sucking noise comin from my tank which can only mean one thing. I have a vacuum in there, but what and were do i check, i do a lot of mudding so maybe i clogged the release valve i also hear something clunking under neather me when i hit a bump but idk if that related or not

 

The vapor recovery system applys a negative pressure to the gas tank, so that it can pull & store the fumes into the charcoal canister for use in starting & running the engine.

SO, if you don't have any vapor system trouble codes, or aren't having engine stalls, what your hearing when removing the gas cap, is likely normal.

If your having engine stalls after driving 10-20 miles, suspect an actng out vapor recovery system, or a faulty gas cap not properly venting the tank.

Not sure about the clunk, maybe time to crawl underneath & have a look-see!!!!

 

Alright yeah ill take a look see. But my truck dus sometimes stall when its cold but thats becasue i have my IAC valve unpluged because its fried