I had same sound during accell only. All exhaust components were tight, but when under accell one of the heat shileds would come into contact with another part and the resonance (rattle) would appear.

The dealer fixed by welding heat shiled "up out of way" of contact area. This of course put undue stress on the exhaust and I developed a leak (just a week after rattle fix). Of course the dealer didn't admit that the cause was there welding the heat shield "up out of the way", but none the less the leak and rattle are still gone after a year and 10K-mi.