2002 C3500 HD 6.5 noise.
 

I have just bought a 2002 C3500HD with the 6.5. The odometer shows 60900 mile on it.
The truck is equipped with a crane so I am not sure how many hours it has spent at idle running the hydraulics to power the crane.


The engine has a noise at 1500 RPM under light load (city driving between 5 to 35 MPH).
The noise sounds like detonation or something like that in the exhaust.
It goes away with more throttle (2000 RPM)


I have replaced the following:


Balancer and Crank pulley. These had not failed but looked like it would in the future.


Motor mounts after one shop that was supposed to scan the truck and trouble shoot the noise told me the fan was hitting the shroud.


Vacuum pump was removed and a boost gauge and mechanical waste gate was installed. This caused another issue with the performance of the truck and a code P0236.


I looked at the flex plate bolts that hold the converter to it. They are tight.
Are those the flex plate bolts that come loose or is it the ones that hold plate to the crank.