Welcome back!!

The alternator is dead... long live the alternator.

When they start humming, you can be assured that the voltage regulator, the diodes or both have died.

One more trick, restart the engine, put a long screwdriver on the chassis of the alternator and put your ear to the handle. Listen... is it really loud? The answer will be yes and that goes along with... The deduction is simple... noise? check. Low voltage? Check. Buy new alternator, or, get yours rebuilt by a good local shop (they still exist!).