Sounds like the bearing in the compressor clutch. This would explain why it happens inconsistantly. The compressor cycles on and off during operation. The clutch can usually be replaced without having to replace entire compressor. It sounds like you have a good idea when this noise happens. Mechanics use a stethoscope like doctors. This helps locate those noises that are hard to get to or locate. Something you may be able to use is a 1x1 board. Wood transmits vibration and noise pretty well. If you can get a board as close to the front of the compressor as possible, start the vehicle and listen. I bet you hear your noise very well. Then just for comparison put the board on your alternator or power steering pump. Remember that there will be moving parts so be very careful you dont hit anything or drop the board in the moving parts. This is a trick I used before I got my stethoscope. I hope it helps.