Is it only when the car is moving, or is it also when reved sitting still?
Really need to get the bonnet up and have a listen, it could be, in no particular order, a failing-
alternator
power steering pump (check the fluid level too, if required make sure you use the correct hydraulic oil to top up)
auxiliary belt tensioner
air con clutch
These can all be eliminated by removing the aux belt and running the engine for a short time (a few seconds should be enough, certainly no more than a minute).
Could also be cam belt tensioner or idlers, or water pump. These are more serious, if they fail they will throw the cam timing out and will be the death of your engine.
There are other possibilities, but noises can be hard to track down when the car is in front of you, even worse over the net.
fred, April 2011
