It sounds as if you could have a gearbox model with the alternator doing double speed, if it is still with just grease in it you can expect this to happen.
Or remove it, open it up clean out the grease drill a hole at an angle to end up in the G/box section using a rubber stopper to seal the hole
REASSEMBLE IT AND FILL IT WITH A LIGHT GEAR OIL USING THE HOLE TO CHECK THE DEPTH OF OIL. I USE THE G/BOX DIP STICK AND HAVEN'T TOPP ED IT UP FOR SEVERAL YEARS BUT IT'S DONE ABOUT 8k miles since
I always use as much lighting as possible LED of corse to relieve the regulator of too much heat dissipation as an alternator with magnets produces current weather it is used or not if not then the surplus is used to produce heat, I have heated grips that are on most of the time as I also have a volt meter indicating when I switch on battery level about 12.5 when started as use an electric starter let it rise to 14,5 V when the amp-meter reduces to zero I switch on my heated grips if my hand get too hot I switch on my tungsten filament head light as I run two with a continental dip and a British dip keeping volts to 13 or just above.
One day I will get round to fitting my Nippon Denso alternator with built in regulator with 30 amp out put but must be geared up so it will charge at tick over like a car and will regulate with out the necessity to produce un-required heat as it will have a field regulation.
best of luck,
bananaman.