Grinding/rattling while decelerating or coasting at highway speed

Grinding/rattling while decelerating or coasting at highway speed.

Below are the traits:
-2015 Mack with automated trans
-only happens when empty
-occurs while coasting or decelerating at highway speed (mostly in 12th, less so in 11th gear on a 12 speed)
-can feel grinding in my feet
-will smoothen out once the truck begins to accelerate again
-putting the truck in neutral causes it to stop
-rattle often progresses to a grind when the truck is giving the slightest bit of throttle so that the engine starts to minimally build but not enough for the truck to start pulling/ accelerating again
-sometimes when hitting throttle to accelerate, it will hesitate and grind for a split second before engaging and smoothing out
- doesn’t happen every empty trip, but has been occurring more frequently by the week and now occurs 60+% of empty trips
-rattle sounds almost like an older roller coaster being pulled up the initial hill by the rattling/clunking chain
-grinding has a higher frequency similar to a vibrator plate
-all of it sounds/feels like metal to metal

I am thinking maybe flywheel, trans, driveshaft joints/yokes or rear differential?
Any thoughts? I have a dealership appt this Wednesday but am hesitant to bring it in without a clear direction of where to look or a guarantee that they will be able to duplicate symptoms.

Thank you,
Pat

 

thanks guys. The leveling valve was replaced at the Mack dealer in late december and they specifically stated they spent a couple hours making sure the ride height and drive line angles were in spec, so I assume they are still good but will be double checking.