This is tough..!! The 620/630 have a whole set of Natural sounds, from those huge parts..especially when first started cold..!!!
It is not a loose setting on a Valve, is it..? Could be piston slap (when cold) or even a wrist-pin with just a MINUTE amount of slap..(if it is, you may NEVER find it!!).. Flywheel SPLINES AND CLUTCH-Driver FIT are Critical and MUST be Tight, with NO wear.. The Hydraulic pump and Governor gears can contribute to significant noise..
If you measured and confirmed the Bearing Clearances, you should be in fairly good shape, internally..