Squealing noise on a 1999 E36 Compact?

I have a really bad sqealing noise from my engine bay. It's really high pitched and can become VERY loud at times. It only starts after about 7 - 10miles of driving. It's at it's loudest at idle and eases off when accelarating until it eventually stops at about 2500rpm. I have taken it to a couple of garages and they can't seem to find where it is coming from. The car has just had a full service. It is a 1999 BMW E36 316i Compact 1895cc.

The noise doesn't change pitch or volume when turning the steering wheel or dipping the clutch. The car does not have air conditioning. The belts have not been replaced but have had a brief check and sprayed with oil as a quick check, so although it still maybe the belt it is unlikely.

Any ideas of what it could be would be great! Thanks!!!