Need help with fluttering like noise.

[K20wagon]

Ok my 02 ep3 bout 5k miles ago it does this fluttering like noise. Its kinda hard to explain. It only does it like after 3k rpms untill bout 5k rpms and it when you half throttle or more and it is getting annoying. My mods are SRI, DC race header and Apexi noir exhaust. You can hear it with windows up and down and sounds like its coming from the engine bay.

[urthworm]

my car flutters like a blow off valve in first and second gear when i let off full throttle.. everyone i race thinks its turbo. dam near half to open the hood to prove its not turbo anymore. But back to your question, im not sure what that could be, does it sound like a bov or more of a sputter? im thinking mine might be the springs on the bc race header vibrating......

[K20wagon]

Ya its kinda like a bov you could say. It does it in every gear though. I kinda thought it was like an exhaust leak and changed all the gaskets but was not that. Then maybe the air filter but nope. Now I am kinda thinking its my flex pipe on the race header. I do have a 2 cels but I have had them before this started. The first one is for the o2 sensor cause of the race header and the other is vtc phase gap that cant figure out how to fix.

[K20wagon]

Does anybody know what my problem is.