subscribing...same damn problem on my 03 P5 with 50,000 miles. I have no idea what the hell it is. And its only when I am accelerating under load. No sound whatsoever when in neutral or parked and revving it. I can't even tell WHERE its coming from. If someone has figured this out PLEASE POST.
Things I have done to try to figure it out:
1) removed VTCS butterflies and checked for loose VICS butterflies (none were loose)
2) running synthetic oil but it doesn't seem to have helped
3) checked for exhaust leaks, none to speak of (except that weird gap between the Mazdaspeed Sport Exhaust (axle-back) and the stock J-pipe which is caused by the crappy, mis-shaped gasket, but I don't know if that is actually leaking)
4) flushed intake system (just throttle body and intake manifold)
5) changed spark plugs
Any other ideas? Next on the list is to change tranny oil to Redline MT-90. I'm also getting a Racing Beat Cat-Back (should be here Wednesday) which should tell me if it was a weird leak caused by my Mazdaspeed axle-back.