There can be two reasons for this :
This may happen due to condensation in the engine. Primarily the reason for condensation is either the change in weather or you are not riding your bike enough. If you ride very short trips then your bike engine will not get enough time to burn off the water vapours. These vapours over the time will create a milky sludge the will accumulate.
The other reason is that coolant has mixed in with the engine oil. This mixing may lead to head gasket leak or even engine damage. Because with mixing up of coolant, engine oil will become contaminated, which significantly will reduce the ability of engine oil to provide lubrication and subsequently will lead to engine failure.
yes , i ride for short distance , and it's cold in my country now ( about -2C ) but i will change oil today , on evening , maybe a oil milkshake can't circulate well in system , and engine don't work properly , that why the noise is ??? i will try . after that i don't even know there the problem could be ???