Irregular Vibration, noticeable at idle

2003 CLK 320 - about 130 k miles

Feeling a small irregular vibration through the seats when sitting at stop lights and
idling. Have had the motor mounts replaced, but this didn't really help. The vibration is
variable and inconsistent - the intensity of the vibrations come and go. My mechanic
wants to run test on the individual cylinders, but the tach holds very steady, so it don't
think it's an engine issue.

Moving the transmission from D to N does not change the nature of the vibration.

Once the car is moving, I can't tell if the vibration goes away, or if it's just masked by
the movement of the car.

Not a big deal (at least right now), but I know that it shouldn't be happening. It started
gradually, but is getting more and more annoying.

Any idea what might be causing this?

Thanks for any comments!