2k Winnie - Beeping Noise From Engine.

Here's the problem. I have a 2000 Windstar SEL, (3.8L). I have a continuous beeping sound coming from the engine compartment. It's not a whine, it really does sound like a beep. It sounds like it's coming slightly to the left of the engine (listening from the middle, over the engine).

I thought maybe it's the serpentine belt, or a bearing from one of the pulleys or the tentioner. But I would expect that sound to be a whine.

The beeping is not there after the van has been parked overnight, it starts after about 5 to 10 minutes after engine start. It goes away when I put the A/C or the defroster. I suspect the beeping goes away because of the additional engine load.

Has anyone come across this before?

I'd appreciate any ideas.

Thanks.