My set up is 2RC intakes, Invidia HFCs and ARK GRIP exhaust. After the installation the exhaust sounds nice!!! love it but when you drive it and go above 3K RPM the whistling noise gets louder. The higher the RPMs the louder the noise gets.
I've read lots of this with hissing problem with newly installed HFCs but maybe this whistling noise is same as ppl describe as hissing.

I did tighten everything pretty tight and used the new gaskets that came with the HFCs. SO I don't think it's exhaust leak, but who knows.

I hope it goes away after time, at least a little. lol

Any experience who had this and did it go away?

 

Sounds like a leak if it's a whistle. Check around the welds to see if there are any small holes. And usually the hissing comes after letting off the gas.

 

Yeahh I checked carefully today after driving. The sound comes not from the HFCs. It actually comes from the exhaust where the mesh is by the X location. Really weird. I noticed that MANY G owners experience this with their newly installed HFCs. Im guessing this is normal. Sounds like a blow off valve lol