Peogeot 206 Y reg, noisy gearbox when not accelerating?

If i take my foot of the accelerator and let the car coast in any gear there's a kind of bearing(ish?) sound. I have tested this with the clutch down and the sound goes away so presume it's gearbox. Is there anything i can do to try and get some use from the car without putting a replacement gear box in? I was thinking maybe adding a very thick oil to the gear box?
Thanks