This has to be the a-pipe and there is actually an easy fix to this. I had the same problem. The a-pipe is double walled and the inner pipe weld comes loose and causes the inner pipe to vibrate against the outer pipe at 3-3500rpm. The area to target to fix this is about an inch past the bend on the a-pipe. I had a muffler shop burn a hole in the pipe at this piont and then refill with weld material (sauder?). The noise was totally gone. Now...I guess their weld has broken down and the noise is back. I contacted an Acura mechanic and a fix that he uses is to simply take a self tapping metal screw and drill it up into the pipe at this point. I will try that next.