I don’t think that’s a noise from the engine itself, it should change with rpm if so. Maybe someone else here might have a better idea. Another idea, have your tried running the engine without the drive belt on to see if the noise goes away? HVAC system working normally? Nothing caught in the fan by the passenger foot well?

 

I know you have owned many Disco's but for what it is worth, the general consensus from folks on this board and several Rover experts is that synthetic oil is not well suited for theses beasts. The engine design lends itself to oils like Rotella 15-40 conventional.
I thought I heard every Disco sound there is but yours is a head scratcher.