weird noise at idle?

[kjmracer14]

i had just started my truck and it was sitting idling in my driveway for a few min then out of no where it sounded like a loud constant whoosh of air coming from the exhaust. I have no cat or muffler just a straight pipe but i have never heard it do this. any ideas? i just installed a dp tuner yesterday and everything has been working fine. When i heard this noise i shut the truck off and restarted it and the sound was gone.

[Jake]

Its your exhaust back pressure valve. Its a butterfly valve right after the turbo outlet that closes when cold to quicken warmup times.