2002 Volvo S60 2.5 non turbo

About one month ago I noticed this steady buzzing sound (no.. not bees) coming under the hood somewhere. Present with car started or just ignition switch on. Seems to be a steady pitch.

For those electrical inclined, does it sound like a rectifier (diode) problems and if so I am willing to repair it myself if not too involved.

Thanks to all!

 

a rectifier diode would not be active if the car were not running and thus the alternator not putting out....