I recently had the same clanking/crunching issue with my Perkins
M30/Hurth 50. In gear at low RPM's it sounded like something had broken and was being tortured in the transmission. Or, maybe the cutlass bearing. Or maybe a loose shaft anode demolishing the strut. Definitely not the damper plate, because it only happened in gear. But at full speed, everything sounded fine.
After draining the ATF, I found gold glittery particles suspended, and some grayish flakes at the bottom. A Hurth specialist anticipated what I saw and told me over the phone
that the contents were normal.
Then, he said a poorly running motor can make it sound like the transmission is eating itself. I was skeptical.
He told me to loosen my injector nipples one by one and see if it makes a difference. Turns out one of three cylinders was not firing, so I had the injectors tested. One was not functioning, and two were really bad.
$126 later, I had three rebuilt injectors, a perfectly running motor, and no more crunching/banging down under.
Hope that helps. Good luck!