Fluttering noise
 

I installed a "K' and "N" air filter the cone shape type it makes an awfull fluttering noise when I let off on the gas any one know how I can fix this or do I have to go back to the stock air filter,.. "yes I have a turbo car

the fluttering is compressor surge from your turbo. as you shut the throttle body, the boosted air has no where to go, this is where the BOV vents it back into the intake. if you have a BOV and haven't done anything to it, then it's the bov just venting back through your intake. if you don't have one, then it's compressor surge, and thats not a good thing, as can damage your turbo.

Not to be a terd but just for clarity, BOVs (Blow off valves) Vent to the atmosphere, and a Recirculation valve, vents to the intake. Should sound like, shaaaaa shaaa sh sh sh. Perfectly normal.

I had the same problem. I just re-oiled the K&N filter the other day and when I reinstalled it, I didn't hear the fluttering anymore. I think the seal around the base of the filter with the MAF is better now. I hand tightened the screw and it felt like it went on more snug this time.

Cant k and N filters damage your AFM, or MAF?? I heard thats what there best at doing.

try the blitz intake filter, its stainless steel and to clean it all you need to do is rinse through with water.... ive noticed that it sounds alot nicer compared to other intakes

That sound is normal with the stock by-pass valve. As far as damaging the AFM, I don't see how as all the intake is doing is allowing more air to be sucked in and the AFM is only metering the amount of air that enters inorder to let the ECU know how much fuel to add.