I have the same noise, like slurping water, when the heater is turned on. I've used the heater alot this winter and I don't seem to be losing coolant.My w124 also has that sound of water coming from the heater inside the cabin. Heat is ok though and there are no discernible leaks anywhere so am waiting to see what happens next.
For now it just puzzles me big time! [:0]
Does anybody know if this sound is normal or if something is broken?