Check the AC Clutch again with the belt off. Check the air gap distance, pull the pulley forward toward the clutch plate, see if it moves enough to touch or at least come closer to the clutch plate. If the noise goes away when you engage the AC, and comes back when you turn it off (remember defrost and recirculate air will turn the AC on automatically), that is a good sign the clutch is rubbing when it shouldn't be.
In my case at least, and granted my AC compressor seized and the clutch glowed red hot, doing a lot of damage to the clutch and bearing. Once I disconnected the electrical power to the clutch, the clutch disengaged and I continued to drive without AC. A few month later I started to get noises that you described, the bearing on the AC clutch had worn down to the point the AC pulley would tilt and walk enough to touch the other clutch plate and make the noise. With the belt off I still could NOT feel a lot of play in the pulley or its bearings, but that gap in the AC clutch is only a couple of thousands of an inch.
Again, my AC clutch was damaged, but its NOT unheard of a normal AC clutch bearing wearing and allowing the clutch plates to contact and create noise.
Water pumps are another one, they will start to leak a little if the bearing is going. Often the shaft wobbles and either damages the seals or lets some coolant past the seal. So if you've got coolant weeping from the water pump, I would suspect it.
Unfortunately, bearing can make no noise at low speed and no side load (like spinning it by hand with the belt off) but howl like a banshee at higher speeds with a side load on them (like with the belt on with the engine running). Alternator, AC Clutch/Pulley, Water Pump Bearings and Idler/Tensioner bearings can all do this.
Last edited by Mongo; 11-25-2014 at 11:25 AM.