my spt make those flapping sound(sound like a fart cannon), everyday in the morning when its cold, and also when my rpm pass 4500 in every geared.....or its normal. invidia catless dp+spt (leak?) thx in advance.
When it is cold in the am, mine makes a little extra noise because of the air pump for emissions control. It is designed add oxygen to the exhaust to further burn unburnt gases from the engine. But it basically pumps extra air into your exhaust and that makes extra noise, specially if you don't have a cat or resonator to further dampen the noise. Do a google search on emissions air pump and there are a few sites that explain it.
I would check for leaks by the downpipe area, and really make sure everything is torqued properly. Not sure if this 100% applies here but when my brother installed an Invidia dp, he used 2 gaskets where the dp meets the midpipe in his 05.