Bad mileage on my 2004 Yukon

[MrJuanderful]

Every symptom you describe is what happens when cats come apart and get lodged in the exhaust pipe and muffler.It can also be an intermmitant thing,as that debris moves around.The cats are the high $$$ part,so if that has already been replaced properly,you can just replace the muffler and inspect the pipes.This is pretty common and will happen to everyone sooner or later.

 

[SC88]

For the Rattle in the exhaust there was a TSB in 04' the welds on heat shield for the muffler would break at a couple spots and cause a rattle, I have that issue now on my burb could possible be yours.

Have you checked the fuel pressure or for any intake/vacuum leaks?

Did you ever do a tune up?

One thing I've learned from buying used cars is never trust the previous owners maintenance records, start fresh with your own Plugs, wires and Tb cleaning is where I would start!