99 Jeep Cherokee 4.0 I-6 auto w/ approx 140k miles

On normal/fast acceleration from a stop, loud, rapid clattering noise from engine. Goes away if gas is let off, or once Jeep is up to speed. Will not make noise if accelerating slowly or not in gear. Have noticed minor power stalls with noise. At first suspected transfer case, but logic points to exhaust system, and transmission recently serviced.

Just replaced freeze plugs (5 on block) requiring removal of intake/exhaust manifold, air, & aux fan. First tried steel plugs (poorly machined and wouldn't fit) had to order brass and plugged like butter! New Fel-Pro manifold gasket had misaligned exhaust ports (0.25" extra on bottom of all six cutouts) ordered and reluctantly used a Victor gasket (same .25" difference/problem). Recently had transmission flushed and filter changed. Repairs (w/in 24 mo) include new alternator, patched a couple vacuum hoses (cruise ctrl and intake), new starter.

Is it the misaligned gasket? Is it something else? Help!:banghead:

Try "better" gas...higher grade
Exhaust header cracks and makes noise...driver's side, before the joint where it goes over to the other side of vehicle.Edit: Your question about the gasket...Use a stethescope...or just an 1/8" rubber hose in your ear.....and listen for exhaust leakage. That header is hard to seal.