cool hand tim

over the last few days i have had a weird noise whilst i am driving, it sounds like a whiring noise but it is not all the time. i don't get it in 1st or 2nd gear just 3rd 4th and 5th, it also stops when i come off the excelerator and the faster i get the louder it gets but there is no drop in performance that i have noticed can anyone help as i haven't got a clue!!!!!!!!!!!!!!!!!!!!!!!