Pinging/Detonation after restarting truck after it's already warmed up and driven

[Metalhead]

I've got a pinging/detonation sound coming from somewhere in the exhaust system, somewhere forward of the cat. This only happens when the truck is restarted after it's been warmed up and driven already, or after it's been sitting idling for a while after being driven. I can give it some throttle 2-3 times, and the sound will go away. It's loud enough that it gets attention from people walking by sometimes.

Any ideas of what this might be?

This sound has been intermittent ever since I got the truck worked on at the dealership for the hi-pressure oil pump failure.

 

[WD40]

Take it back to your dealer and have them check this out, but it may be a bolt on the down pipe that's sticking out just a little to much on the down pipe. Mine only made the sound at around 27 to 33 MPH.
Hope this gives you somewhere to start.
Doug