Combines knock

[ski_whiz]

Be sure the noise is not in the main output shaft as this turns with engine. Separator clutch pulley turns as well while cranking. Any klunk in the flex plate or splines may disappear once under load. If you're certain it's in the engine, check the timing geartrain. Is the noise at the front or rearIJ

 

[R_J]

I have unhooked the main belt at the engine so all that is turning is the engine. The noise seems to be at the back.

 

[ski_whiz]

Check end float on output shaft, should be zero. You could remove the output shaft - not a bad job after removing seat and opening access door. After removing you could peer through the hole and inspect flexplate, splines and bushing. Crank engine without shaft installed - if noise is still there, Must be internal engine trouble (rare).