1996 K1500 (with rebuilt tranny and 2 flowmaster mufflers w/ hand-bent 2.5" dual straights 2 yrs. ago)

I've been hearing a noise that, to me, sounds like its coming from directly below the passenger's seat or maybe just above where the tranny sits. The best way I can describe it is:

- it only happens on acceleration and definitely gets louder the more gas I give it

-even though it's an automatice, it sounds like what a manual transmission sounds like when it's in too low of a gear - when it is shaking and about to stall out like its fighting to stay running. (I referred to this in a previous post as "lugging" the tranny but I don't know if anyone else uses that term)

Any help would be greatly appreciated! Thanks in advance - I know these 'noise' questions are tough to diagnose over the internet.

My only guess would maybe be an exhaust manifold/header or cat problem? If so, where do I start?

Try beating on the cats with your hand to see if they rattle. I'm sure you already know, but make sure they are cool to the touch.

Could be the donut gaskets too and are a very common problem.Most parts stores stock them.