B5 s4 noise 2 - YouTube

What is it? I know it has nothing to do with anything on the accessory belt. Is it a boost or vacuum leak? Any help would be greatly appreciated!

 

Jesus christ that's horrifying

Judging by the sound of the rpm's as your buddy was revving, it sounds like it only happens on throttle, is that accurate? Do you have a boost gauge? Does it happen under boost and vacuum or just vacuum?

 

I know this is terrible and I would be mad if my car made that noise, but as a listener, I have to admit is sounds funny.