When I first heard the sound it reminded me of what "floating the valves" sounded like from my old V8 days. Many older hot-rodders may well remember that sound coming from spinning a 350 small block to 5K rmps with stock heads...
Floating the valves is a nasty metallic sound with accompanying backfires and smoke... ultimately where the valve train is to weak to let the valve fully re-seat before the cam/pushrod was asking it to open again.
I don't think this is the case with the FA20 - there are no backfires or smoke or any other outward appearance beyond the noise during the particular rpm range. From my point of view - it is nothing more than harmonic sound distortion. As I said in my original post way back on page one.. the rasp is gone post header installation as is the infamous dip.
That said, If I were concerned a tune is causing the issue... I'd convert back to stock, or find someone with a tune that does not introduce the rasp. (But I'd have to agree with some others on here.. if you want the dip gone, deal with rasp or get a header...) /2cent
