Had this problem it’s the trans rattling because of “torsional vibration “ caused by the engine .
All Prima M50’s do it . The standard drive plate isn’t effective enough to stop the engines TV . I replaced my 25 yr old Hurth
HB10 with a new ZF 12M and a high quality new drive plate with no springs in it and it still chattered .
I did run the engine for an hour “without the trans installed”
and the engine ran silky smooth absolutely no rattle .
The engine’s TV causes the trans to rattle after warm up .
A good quality drive plate will stop it a little.