When I had my wife shut off my truck while I was under the hood admiring my new 4" exhaust, I heard my air box keep sucking air. I had her do it a couple of more times and the same thing happend. 2 or 3 seconds of air movement. It may have always done this and I never noticed. Does this sound normal?

 

This is normal. It is your turbo spooling down. You should be able able to here it at your exhaust as well. I counted mine one time and it took almost 7 seconds before it stopped.

 

Just imagine if you had just reved your truck up and shut it off how long it would turn, and how hard it would be on the bearing with no oil moving!!! I seen a guy who works on diesels in Atlanta do that very thing with the air intake off, he was playing with the govenor and started it up reved the crap out of it and imediatly shut it down you could see the turbo spinning. I decided I didnt need him to help me with my truck.