Popping , grinding noise from transmission

Dealer reported : need tranny replacement .
Short story : yesterday driver called and reported via txt "It won't build up air so I can go I'm stuck in the middle of the street ima call tow truck " , I called tried to figure out where the problem is , compressor seems fine building pressure but as soon as governor cuts off and driver release either truck or trailer brakes pressure drops to 70 ( per driver)...driver couldn't do anything , got blocked on the bridge somewhere in downtown charlotte and wrecker arrived , tow truck called me and asked me if I wanna bring the truck to his shop for transmission repair , I politely refused smelling bs and ask him to bring the truck to peterbilt dealer 20 miles away , 2 hours later I got a call requesting to pay 2300 cash only ( I was in Chicago ) otherwise truck goes to impound , towing was working just shy of 2 hours ( flat top Pete with loaded stepdeck ) , asked the driver to play with the truck on the parking lot to figure out where is the air leak , apparently no leak but when he puts truck in gear - shifter jumping like crazy per driver . Today dealer inspected the truck and found 0 air leaks or issues but reported tranny issue : popping and grinding noise when in gear ( maybe when not in gear as well ) , they recommended replacing transmission , can't say what the problem is without removing it .
Transmission been rebuild by very reputable shop locally in Chicago October 2017 and I personally drove the truck from October to April for about 60-70k miles and maybe 5-6 during summer months without any single issue , flaw or hiccup .
Driver started to drive the truck 6 weeks ago and drove probably 12k miles , before truck was given to him everything was checked out perfect with all fluids good , no leaks or issues . Today driver refusing to admit that he reported air issue " When I say issues I mean no air leak It would build up but when i push in button it lose air but both of them are saying transmission " ( now the meaning - tow truck and dealer both said tranny issue ) .
Not even sure what is right in this story , kinda have a feeling that driver messed up tranny ( before he drove 10 speed , mine is 13 ) , dealer estimated 8k plus so I'll be coming for the truck and bring it home for repairs ....just wanna hear thoughts and advices on this situation
...almost forgot : driver said no issues noticed lastly with tranny , even yesterday morning

 

2 years experienced rgn driver , 25 years old

 

Dealer would figure ...no low air pressure or loss in air system
Dealer said clearly : popping and grinding noise from tranny