If anyone is experiencing an engine rumble that reaches a pitch,creates a vibration,doesn't change with any sort of fiddling ie timing adjustments, throttle manipulation,but is most noticed on start up and above 2000 rpm and changes little with load, check your crankshaft pulley for being loose or damaged.
Bent,(Sounds somewhat like main bearing rumble but not quite)
Grooves for V belt distorted bits broken out,keyway loose.
as I replaced mine and the rumble is gone.
It had distorted V belt grooves and was a fairly loose fit on the shaft.
See photos it appears as if it has been dropped onto a hard surface or hit with a hammer although the damage is minimal it kicked up a fair rumble as the belt passed around it. :blink:

It is soooooooooooo cool to help others out of the blue :D

Now I know what to do if I experiance this problem.

Thanks