I would pull the serpentine belt and start spinning things by hand to see if you feel anything in the bearings. I also have great luck with using superlube ( not the kind that most everyone is thinking about, but the industrial spray ) and carefully lubricating a bearing while in place. Then start the engine with everything put back together and see if the noise changes. When you get one to change the noise then that is your culprit. Please do not get the spray on your belt or the belt surfaces. If you do, don’t start the engine, spray the offending area with brake cleaner first. I probably wouldn’t try and lubricate the rear alternator bearing but you can do the front one. It could also be a bearing in your distributor, so don’t forget about that if you can’t find it in the belt system. Be careful when removing your rotor, I don’t have Rover distributors but apparently there is something that people mess up when doing that on a genuine Rover dizzy. Good luck.