Good afternoon, I'm hoping some one can help me or point me in the right direction, I have an Audi a4 2.0l TFSI Quattro and I'm getting a really strange noise from the engine, when running a idel it sounds fine but when it is revved up and as soon as it passes 4000 rmp the engine tone changes, ( it sounds like I have a blown/missing exhaust) it alternates between the two tones, normal and quite loud. When you then start to bring the revs down the engine tones( loud and normal) seem to slow down and the at around 3000 rpm it will stick on one of the tones, and then sort of seem a bit sluggish. I know it's hard to describe in writing but it's really starting to annoy me. Also vag-com isn't showing any thing up on the engine, help please. also I have had the PVC valve changed, diverter valve new cam shafts have also being fitted along with chain, tensioner and adjustor.