shake

All, I hope you can help with what might be the culprit of a very annoying problem. When I drive the E220d estate (2016 plate), I can feel a very slight steering wheel shake/vibration. I've had it in a couple of garages, and no one can seem to find the fault. They say they've tested the bushes...

My E280 CDI (2007) shakes gently. Whilst stationary the steering wheel vibrates slightly and the car shakes (similar to bad wheel balancing) when travelling above 40 mph. At my last service I was advised that there was slight engine mount leakage and i'm now considering getting them replaced but...