Drop the oil and inspect it, see if there are any tell tale signs of things that shouldn't be there, particularly out of the sump drain. Also take the clutch side cover off and take a look at the primary gears.

It is not unheard of for the crank nut to come loose making horrible noises at first and then, if not sorted quickly, will ultimately work loose enough to mash the primaries and possibly lead to premature main bearing failure.

These are both easy to check with the engine in the frame and worth doing first before completely stripping the engine.

__________________