C-15 Grunt...?

I've got a 05 freight liner with a CAT C-15 651,000 miles. It make a grunt or quick grind or rubbing noise when at idle, it's quick and I can feel it in the floor board. I happens every few minutes after a pull not when I first start it. Any ideas on what I should look for besides go to CAT. Any help would be greatly appreciated. Thanks

 

yes I opened the hood! It does sound like it's coming from the rear of the engine or the tranny, but only in idle.

 

on highway about 50-60 mostly 52-53

 

I literally just had a PM done at CAT and oil sample and everything came back clean. That was about 1,500 miles ago.

 

Yea that's what I'm thinking, I thought I would throw it out there see if anyone else has heard or know of a similar situation. It's a 18spd.