The gurgling noise you hear is not normal. It's caused by air trapped in the heater core, but don't worry, there's an easy fix. MB Service Information bulletin 83/33a recommends installing a "gurgle kit" to eliminate the problem. It's a simple DIY job, and it works. Back in the good old days we did lots of them. When you go to your local dealer to order a spare key, ask them for a copy of the bulletin. If you decide you'd rather live with the "gurgling", don't worry about it. It won't cause any problems that will cost you in the long run.

If you want to try and fix it for free, try this. Raise the front end, or park on a steep incline. Remove the pressure cap and run the engine at around 2,500 RPM, with the heat on MAX. for about ten minutes. This should allow most of the trapped air to escape. This is what we used to before the "gurgle kit" was available.