2002 Renault Megane Strange noise coming from gear box

My Diesel megane has developed a noise that only happens when I am reversing the car or slowing the ar using engine braking in 1st, 2nd and 3rd. can anyone enlighten me what may be the problem. The noise is very difficult to describe but the best way to describe it is a low pitch almost like a grinding noise but i dont think it is grinding.
Thanks