Ugh, P0420 on my 2nd set of cats

[NGAneer]

By the way, a bit more facts for those of you wishing to know and who actually care.

Yukon has 144K as I originally posted. I replaced plugs and wires at about 120K. Air filter is clean.

HOWEVER, this motor does an odd thing that I really am baffled about--and I struggle to repeat what it is. Let me try to explain it and tell me if you've heard of this before.

Again, can't repeat it at will all the time--but when the motor is downshifting, say up a slight grade and I give it a bit more gas, just when the pause happens in shifting and the revs are about to jump, you hear the "PHHHHHIT" sound. Take your lower lip and cover half of your top teeth and blow hard. You'll make an odd air noise. The motor will sometimes make that for only about .25 of a second. My wife has looked at me with that "what was that face" several times and I've often thought it sounded like exhaust just blew back "somewhere". I know a great deal about engines and can't imagine where blow back would happen--nor how. I don't consider it an exhaust leak because I would hear and exhaust leak all of the time--and not only on rare occasions....RIGHT?

TKH

So the motor is loaded up because you're going up hill, right? Assuming yes, I'll talk through my thoughts. It should be in V8 mode. If you give it gas, it's trying to pull air and push exhaust so you've got a higher pressure differential on either side of the cylinder. I would check for evidence of an exhaust leak and if you had a vacuum leak it should throw a code (does take much of a leak in my experience). Is it using oil? Loosing coolant? Just trying to things of anything that might cause that sound.