Maybe belt tensioner... Though, when my was goin out on my ol 7.3 it made a quick squeek at shut down. I took that for a "dieseling" sound until it shredded two or three belts within a couple days. I can't remember hearing and clunk/thud like that before. Then again, I've only been in the game since '10 with an auto 7.3 & now newer man-auto 6.7s & I don't work at a shop. Though, you can check your tensioner by checkin to see if it's easy to move (may be hard to judge) & then taking it off to see if your marks are lined up and if the space is even in between the two main parts of it. There's a big spring inside the two pieces and when mine came unsprung, one part was offset from the other. Also, the one mark was close to or right outside of one of the tolerance marks on the other piece. Hard to describe but, easy to understand when lookin at it. I don't think it's the tensioner. But, it's easier enough to check out.
- A vet' that's gone from a 7.3 to a 6.7, tryin to gather info & help out when I can. New to the 6.7 & PSA. Please excuse my potential FNG ways.