When my 300e motor mount went out, the car had a very noticable vibration at idle, almost like an ignition miss. In motion nothing felt unusual.

The mounts are glycerine filled and when the fluid leaks out they collapse very slightly lose their ablility to absorb vibration. I noticed some grey colored fluid on the garage floor.

Changing the right side mount on my 300e was very simple. The V8 engine might be a different story. My right side mount has failed twice and the left side is still original as far as I know.