Just noticed the sound yesterday, and even more so today with my windows down.
With the engine warm, under moderate to heavy acceleration there is a loud constant hiss coming from, as far as I can tell, underneath the truck cab.
I thought that maybe this was my clutch starting to slip but i can hear no noticeable increase in RPM.

I also am fighting a ping but i am not sure if that is related to this problem.

Any help would be appreciated.

 

I had similar and it was an exhaust leak at the manifold to down pipe flange joint. Sounded like a vacuum cleaner or a Chevy V-8 swoosh noise. One bolt had worked very lose and an other a little loose. There was a small separation. Tightened all three and the noise went away.

 

Cool, kinda what i was hoping, perfect time to cut the cat aswell.

 

Check before you cut.

 

Make sure your air cleaner lid is on properly. If a snap closure is loose, or the wingnut in the middle is loose, it will hiss like a rattlesnake whose tail got stepped on.

I know, it's happened to me a couple times now, especially if I get in a hurry closing it up after working on it.

Let us know what you find
Pat☺