Dunno if I am being paranoid, but I am wondering if I am developing an exhaust leak. Under acceleration, and more so when going up a hill, I think I am hearing more noise than I should. No noise at idle, or when revving engine while parked. Kind of a chugging noise, not super loud but it is there and I had not noticed it until recently. Seems like it started once the weather got cold around here.
Head gaskets were done about 13k miles ago.

 

Get underneath the truck and see if you can snug up the bolts to the y pipe

 

put a rag over the exhaust pipe, if you have a leak you will hear it blowing out under the truck.

 

And you can do a SeaFoam induction treatment (outdoors!) and the smoke will come out the leaks as well, sometimes the spot is in a place you can't see (back of a pipe, etc.). Will also kill skeeterz within 100 feet. Does not hurt truck, but people think it is blowing up. Makes a lot of smoke for a few minutes.

 

Just took a quick look underneath, looks like where the y pipe connects to the exhaust manifold on the passenger side is missing two nuts! I guess somehow they worked themselves loose. That would appear to be the obvious spot for the leak.

 

Good! Easy fix