2001 Peugeot 307 NOise when revs go up and down onloy in ge

Hi,

My 2001 307 has began to make a strange what can only be described as a whiring noise when accelerating or slowing down. It cannot be heard when the speed/revs are kept constant and it goes as soon as it is taken out of gear or the clutch is depressed. Any ideas on problem/ cost to fix etc?
Thanks guys