Water behind the dash - this is air in your lines. Follow the advice from tim down under (from another forum). I did and it fixed it.

--------------------------------------------------------------------------------

It has something to do with the heater hoses located in that area. Its quite possible that you have air in the lines. Have you had work done in that area?

Some owners have complained that swapping the hoses can cause this problem.

This is quite a common problem in Land Rovers; not sure whether it is the same in MB. In any event you might try this explanation and cure from Land Rover:

Most likely, air was allowed in and that's why you're hearing the coolant slosh around.

Make sure your coolant level is topped up. With the engine cold, fill the reservoir to the top. Start the engine with the reservoir cap off and let the engine warm up. At the same time, turn your heater to the highest heat level and max fan.

Run the engine with an idle speed of about 1500 to 2000 RPMS JUST for a minute or so. Doing this will cause the engine to draw in the needed coolant from the reservoir.

This should fix your water noise problem. At least for a Land Rover.