Alternator humming

85 camaro. 2.8Liter fuel injected multiport 135k on body. 80k on motor.
The alternator I had on there ended up sh**'n the bed. I had one from a 87 firebird layn around. However it had a different plug adapter. So I swaped out the plug with one that fit the newer alternator and now it has a faint hum. Even with ignition off. Do I have something miswired? Car starts up and runs.

ALSO. The battery meter on the dash reads about 13 with just ignition on and about 15 with car running.
The hum is real quiet. Need to have ear almost touching the alternator to hear it

if it's humming when the car is off, then there is a compatibility problem. you should change it out to the proper model.