I have the similar problem with my 1999 E320. There is a slight vibration on every 10-20 or so seconds while the car is at a stop light. During driving there's no issue, it smooth as new. I took it to the shop. They investigate (checked by stepping on break and revving the engine) and ruled out engine mount. They suspect Transmission mount which they changed. But the vibrations still persist. I them added fuel injectors cleaner - Still same vibrations issue.
I open the hood to listen at the engine at idle. There are no vibrations but I do hear a hissing sound on the driver side. Not sure what that is or whether it was there all along.
labdag
I open the hood to listen at the engine at idle. There are no vibrations but I do hear a hissing sound on the driver side. Not sure what that is or whether it was there all along.
labdag