Engine noise... backpressure?

[mkory]

Alright, let me try and describe this. When I accelerate I hear a sound come from my engine. It sounds like a puff of air, almost like a blow off valve. It usually happens around 4,000 revs. If I'm driving it really hard, once again around 6,000 revs. It doesn't happen all the time, and I've tried to but haven't been able to find a pattern when it happens. I'm thinking maybe some kind of backpressure issue. Like I said, it only happnens some times. I can also feel it affect the power. When it happens, it's like I lose quite a bit of power(only for a split second, and just enough to feel) then the power resumes as normal.

[miustapimp59]

Try checkin' your AIV Valve, there maybe some kind of leak. Or check for leaks around your exhaust system.