wierd sound(train like)

out of the blue my car has started to make a wierd sound. it sounds almost like a train going down the tracks at high speed. its not loud, and when I rev the engine it doesnt get louder, but does sound faster. sadly I really cant pin point it comming from eather side, you can hear it all the way around the engine, but it is best heard standing directly in front of the enging.
any one have any ideas what it could be?