Rumbling noise

[Lakotah]

Don't know how best to describe the change of sound from my engine but it has developed a sound like what you might describe as all cylinders are slightly not firing 100%. It still responds well as far as acceleration but it doesn't sound normal. I have just over 92k miles and poured some injector cleaner in the gas tank and notice no change. I don't know maybe I should run the gas tank down and change what gas I use which is Shell. I have to mention this has been going on for less than 1k miles. Thought I might ask here before I take my C-5 to the shop.

 

[LLC5]

If the CEL is not on and there are no stored codes, then check for drive belt noise, lower pulley running straight and not moving on the hub, exhaust and muffler leaks.