Does it have a high pitch to it? Like if a bird could purr? (Probably the best description I've ever heard). If so, I'm thinking engine harmonics. If it has a tick noise or you can easily locate the noise to an obvious source in the engine bay with a stethoscope, then it's likely mechanical...belt tensioner perhaps. One thing you can do is put a dab of oil/lubricant on a rag and while the engine is running, lightly touch the small oiled spot of the rag to the outside edge of the belt (facing the front of the car), if the noise goes away or quiets: you're looking at belt, possibly tensioner and idler pulley. Btw...I accept no responsibility for anything should you manage to injure yourself fiddling with moving engine parts.