could be either mate ive worked as a mechanic for many years and you will find that 99% of release bearings when knackered will make noise with the foot OFF the cluth and the noise going away when ON the clutch this is due to any play in the bearing being taken up by load being applied to it so they can be heard while driving and engine braking etc.. however when really worn and on the verge of collapse they will make noise all the time but i would not rule out the gearbox being at fault also. the only way to tell is to take the box out and have a look!