Noise when lifting off accelerator

[737-800]

1998 A4 1.8t
After accelerating hard,when lifting off the accererator there is a strange slapping and bouncing noise. It sounds like a butterfly valve bouncing shut.
First I thought it was the throttle body now im starting to think its related to the turbo.

Any one got any Ideas before I pull the hole lot apart?

[richie_jones]

its the audi dump valve venting air into the air box, hear it more when you`ve got a k+n fitted. don't worry about it, its usual...