Spinning noise under hood

[blahblah95]

When I drive and press the gas lightly and gradually to about 20-30 mph I can hear a high pitched noise in the hood as if something is spinning. Any slower or faster than that you cannot hear it. I have had my mechanic drive with me but the car doesnt wanna make that sound then! But it happens whether its hot or cold and everything including belts have been checked out. This sound drives me insane! The car has had it for about 4 years and no one has been able to figure out what it is. Thankfully nothing has malfunctioned but I am so annoyed of hearing that sound. It is definitely something that spins. It is high pitched but not whiney, clankey just high in sound. Anyone have any ideas what it could be?