Old thread, I know, but trying my luck anyway. I’ve got this exact noise and have done for a while. Sounds like a bearing whining to me. It’s hard to hear the sound externally, but is very noticeable internally with the blowers directed at the windscreen and becomes worse when the A/C is on. Unfortunately my van threw its serpentine belt off last week, I’ve managed to replace the belt and all is well, but the whine is still present. I’m much more concerned by the whine now, incase it was the cause for the belt failure. Found various threads about this noise and none of them seem to come to a conclusion on the cause.
Mines in the garage to remove the belt and check out each of the pulleys in more detail on Monday.