Three posibilities Water pump , head gasket or oil cooler .Most likely the water pump mechanical seal if there is no coolant in the oil.
I do believe thats what I said, water pump problem, if you read both threads quiet when cold, rattles like mad when warm ?? what colour was your coolant last time you changed it?They are related and no you cannot get bearings or seals for it, the complete pump needs to be replaced.Since it does not happen to often it is worth taking a chance on a second hand pump.
mine looked like this! had to double check if it was water cooled or not, it looked like oil and smelt a bit like burnt caramel.
Mine looked just like that 2 years ago when I first changed the coolant after my first servicing. I put it down to not being changed EVER in the 10+ years the bikes been around.Coolant just changed. This time the waste looked much better - no 'oil' etc. Could be wrong though!