I have a problem with my X reg Renault Clio vibrating at speed?

I wonder if any one can help me please, I have a X reg Renault Clio which I use as a second car but recently I’ve notice a bad vibration nose/bit of a wobble that it feels like it coming from the front near side wheel arch when the car is doing about 70/80mph down the motorway. Its not the wheel balancing or distorted wheels as they have both been checked and the wheels have also been swapped for the rolling road test…….They are not sure what the problem is they feel it may be coming from the gear box…….!!! I think it may be the prop shaft ??? All I can say is when it happens (which isn’t all the time) if I dip the clutch the nose stops and when the clutch is engaged again it back!! Can anyone help of have come across this problem before??