Sounds like a pulley. Get a long 1/4" extension(if you don't have a stethoscope) and listen to each pulley bolt head while motor is idling to find the source. For things like AC and alt put the extension on the case as close to the pulley side bearing as possible.
Alternatively, Remove belt and spin each component by hand 1 by 1. Some times you cant spin them fast enough to produce the sound.
Can also get a can of brake clean, with motor running spray each pulley bearing, the sound should go silent one the suspect pulley. It could also just be the belt itself. Inspect it for cracking, it should be firm but not hard when scoring it with a finger nail. If sound goes away when spraying a pulley try spraying a different one. If the sound also goes away on that pulley belt is likely the source.