Exhaust leaks, donuts not sealing from manifolds to y pipe?
Vacume leaks (mine did the same thing last year when the EGR was leaking by, before it rotted off) Was alot louder than normal, bad fuel milage, smelled rich all the time.
Iv dropped a tranny mount from torquing the truck too much, tranny fell down on the crossmember and was making a loud funny noise too.