How can I fix a very loud but perfectly working Clio gearbox?

The car is an 11 year old Renault Clio.There is no unusual sound if I rev the engine while stationary,but when the car is in motion,there is a heavy drone sound which rises and falls with gear changing and acceleration/deceleration.It seems to me that the car body is amplifying the gearbox in some way but I don't really know?