Christian - it sounds to me like the alternator bearing going bad. Something external to the engine. Maybe take off the drive belt and check if the alternator bearing is making any noise when you spin it.
Thanks Cees, I appreciate your input and suggestion.
Unfortunately, the sound/noise is not there when the engine is cold, but only when it reaches normal operating temperature.
Thus, it seems to follow the engine oil temperature, and this was why I was thinking of the camshaft or camshaft bearings. But just a guess......and I have no prior experience of which noise/sounds are associated with a worn camshaft or bearings. But maybe you or others have?
Cheers,
Christian