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Purchased a second hand HKS Jasma 4 2 1 stainless steel header for my na8. Fit them and noticed that my already rattly exhaust now had a "flapping " noise when the accelerator is pushed (mostly under driving load, increase in frequency with more rpm) near the passanger foot well/gearbox area.

Now I thought that the header was just exaggerating the rattle due to more flow but I had my high flow cat fabricated today and my new Racing Beat cat back fitted with has removed the rattle but the new flapping noise persisted.

It must be something in the header. There are no leaks from what I can detect. Possibly something to do with the flex pipe? Is this just what na8's sound like? My na6 was much nicer with a similar exhaust setup.

Any insight would be appreciated. Tempted to fit a oem cat or a test pipe to see if it has anything to do with the cats on the car (old cat was a high flow, so is the new cat).

had the similar issue with extractors i bought, but i got a bigger muffler and new gaskets and it seemed to fix the problem

It made the same flapping noise with the old muffler and the mew muffler, the old muffler was much loader then the new one on the car at the moment (racing beat).

It sounds like a flag flapping in the wind very fast. I am thinking maybe the inside of the flex pipe is separating and moving around under high gas flow at acceleration

revhead wrote:Is this just what na8's sound like?

It's not NA8 specific.

It must be a manifold gasket, or the flex pipe in the header. I'm going back to the exhaust shop today so they can take the car for a drive and have a listen

It could also be an exhaust leak, maybe somewhere along the joints.

Brought the car down to Havoc Fabrication, he pointed out cracks in the welded portions of the exhaust manifold possibly more further down the header

Revhead. Just saw this, I have the same HKS header and the flexi is the culprit. Have to take it out and put in a new flexi.

Turned out to be cracks at the welds on the exhaust flange. Seller took it back to be repaired, seems to be holding up well now.
It was so badly cracked the car run pretty crap. But now it feels much better. I still have my suspicion that some exhaust is escaping at the flex pipe though.

It's liveable but annoying at low revs.