Transmission noise at low rpm and idle

Transmission rattle/clattering started at low rpm like at around 1300, i was empty and it would start clattering for maybe 5 seconds stop for bout that long ect. I could hear when their was very little torque goin on. Also a clammering at idle but when driving it was a start stop deal.
Didnt hear anything when gettin on it

Thanks in advance for imput on this one

15 speed 1571 torque i believe, ive got truck turned up to 600hp, stock was 435, 920k on tranny or thats when some bearings and gears put in from prev owner. REALLY dont want deal with this now, was hoping to get another couple years worth. but i want to have an idea before i drop it at shop thurs. shifts fine btw