I have read as many threads as I can on this and can't really find someone with the same symptoms. I have a revo cai installed and the car has started making this flutter noise under hard acceleration. It usually happens when you accelerate from lower revs and in 3rd. If I go from 2nd at high revs it goes fine. All things I have read say the flutter should happen when you lift off. I only get the big hiss when I lift off and get no flutter. So not too sure if it is actually meant to sound like that or not?? Also, the main symptom which makes me think (hope) it's the dv is the fact it feels underpowered. It is remapped and I struggled to pull away from a 330d the other day whereas before this would have been an easy win... Sometimes it goes as normal, usually when I go from second at high revs. It almost feels like the turbo isn't kicking in for sone reason... Then when the revs get up you can feel it trying to go. Diverter valve? - YouTube That's the YouTube link to the flutter, apologies for the quality, you should be able to hear it and it is a lot louder in person! Also, it is going in soon for a new clutch/flywheel/thrust bearing. Don't know if that would be the cause if this though. Thanks in advance for any help guys.