Check for a loose heat shield. There's several, on top of maniverter, on firewall below maniverter, on the driveshaft center carrier, above the muffler, and I think that there's one more somewhere. The thin metal of the shield rusts to the point that it comes off of the bolts, then it just starts flopping around making all kinds of racket. You usually have to actually push on them to check if they're loose.