My '97 1.9 dti Scenic has a very noticeable vibration between 2300 and 2500rpm. It only happens when in gear and is more noticeable when the engine is labouring. It can be felt through the bodywork and the steering wheel. It doesn't happen when revving the engine in neutral so I don't think it's the auxiliary belt tensioner. It seems it might be a gearbox problem. Can anyone help?