I just bought my 2013 C30. It does a very noticeable vibration at a stop light. I put it in neutral or park and it goes away. Any theories or suggestions greatly appreciated.

 

I'd start with inspecting motor mounts/transmission mounts. Does the idle change RPMs when you drop into gear?

 

 

sounds plausible - you can try listening around with a mechanics stethoscope or similar tricks. Tough to say without hearing the noise and location. Just have a helper drop the car in gear with the brakes on and listen around under the hood, under the mid section of the car (ie for tranny or exhaust noises) etc.