I have a 2001 S4 with the APR chip. In chip mode when I give the car full throttle I hear a rush of air coming from under the hood at about 4000 rpms and I lose some power. My throttle body boot is fine and I took the car to Audi but in stock mode, they couldn't find a problem. Nothing appeared on the Vag Diagnostic test. Anyone know what this could be??? Loose or torn hose for Turbo??? If a hose was loose or torn would a fault code be thrown????

...but hey, it makes sense to delineate forums based on that...NOT.

...just pointing out that there is a forum here devoted to his S4, and he'd likely receive a response there to his question.

Had a 2000 with an APR chip that had the same sound. I took it into my dealer as well and they found nothing. Turbo's eventually blew (about 5000 miles). Don't really know if the chip had anything to do with it but I tried the APR chip again after my turbo's were replaced with the 2001 turbo's and same noise and loss of power so I never used the chip again. Talked to many about this and no one I talked to had any problems with other chip manufacutres.
Anyway,

did you take it off and check? that's where mine was torn.