I just noticed today while under the hood that the alternator is humming while the car is off. What could be causing this?

Wow, I haven't heard of that before... is the alt warm when the car has been sitting overnight? I think there's a resistor that keeps the batt. current from reversing and coming back to the alternator (one-way valve you could say) and so maybe that's gone bad.. Is it the original alternator?

No, I just replaced it after my stock one went out. I'll check it in the morning.

If it is warm, what do you think it is?

If the gen makes noise when it is off it means the field is recieving voltage and it is not spinning. Did you make any wiring changes on the 2 wire plug. If not return the gen, most likely the regulator IC inside the gen is bad. Make sure when replacing the gen that the battery line is fully disconnected. The replacement gens have more sensitive regulator IC's inside and if they are hooked up with power on you can get a condition called "false biasing" which can destroy the IC.

Did you by chance replace the plug on the alternator? I noticed mine doing that for a while. Kept killin alternators that way. Anyway, the plug was messed up. It was jumpin the wires (2 wire setup) so I was gettin that buzzing. Check it out ASAP! The guys at Autozone whine when u gotta get warrenty on those things!:-P

Alright, thanks for the help. I'll look into it asap

heh the guys at AutoZone whine about your whiny alternator...