The engine mounts on a car are the direct mechanical connection holding the engine to the chassis. Almost all engine mounts have a rubber portion that allows a small amount of flex to reduce vibrations. If the engine was just directly bolted to the chassis, you would feel every vibration from the engine.
The rubber portion of the engine mount is particularly susceptible to failure due to the conditions of the engine bay. Heat and oil can take its toll on the mount, causing the rubber portion to crack and separate. When this happens, the mount collapses and no longer supports the engine correctly. Replacement is tough due to the very limited space on the C350. You should be prepared to spend more time than you think it is going to take. Sometimes the 16mm bolt on the top of the mount will line right up and screw straight in and other times you will find yourself raising and lowering the engine trying to get everything to line up. It will line up eventually. Just be prepared to spend the time on it. If you get frustrated walk away and come back later. Don't plan on doing this if you need to use the car in the next hour.
A simple way to test if the mounts are failing is to have a friend sit in your car while it is running. Place the car in drive, and with their foot firmly on the brake pedal apply a small amount of throttle. Then place the vehicle in reverse and do the same procedure. If you notice the engine having any large movements your mounts are in need of replacement.
Other kinds of rubber metal mount are available.
