Inconsistent flutter noise when slightly accelerating
2012 Chevy 2500 HD. 26,000 miles. After driving for about 15 minutes or towing this noise starts and gets louder with time. It is an inconsistent fluttering when I let off the gas for a bit then press back on the gas. I took the truck in and they told me they couldn't find anything wrong but wouldn't show me results of any test run on the truck, said the technician forget to press save on the computer used to preform any tests. Then had the balls to say that maybe I don't know what a diesel truck actually sounds like. Ha!
The noise is non rhythmic, inconsistent, but always there after driving for a bit. Anyone heard of this or point me in a direction to find out? Thanks for any help.