I have an X02 HKS Hi-Power Replica on my '03 Z. When I put on the exhaust it sounded great, but a little raspy as with all HKS replicas. I decided to weld a resonated X-Pipe in to the Y-Pipe, using the same X-Pipe that everyone else uses (Magnaflow 11386). Had a shop weld it in and it's sounds amazing. Just one problem. When I stand still and rev, the exhaust has a high pitched whistle that comes from the tips. When I am driving and go WOT the whistle appears around 4k RPM and lasts until I shift. Four things are for sure: 1) The whistle is not coming from the engine bay, absolutely no intake or vacuum leaks. 2) there are no exhaust leaks in the flanges or welds, exhaust is sealed. 3) There's no blockages in the exhaust, and the cats seem like they are in good shape, power is good, doesn't feel sluggish. 4) The sound is 100% coming out of the end of the exhausts, as in the exhaust tips. Before people start crying about me having a replica exhaust etc... Save it. So, does anyone have any idea or experience that might shine some light on this problem? The whistle is loud and obnoxious and ruins an otherwise perfect sounding exhaust. Cheers!

 

https://my350z.com/forum/intake-exha...-high-rpm.html

There was another guy who had the same issue not too long ago and it turned out to be a small hole in the welds causing the whistle.