SLK 350 Gurgling noise

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When I turn on the ignition in the morning or after the vehicle has been standing for several hours I get gurgling noise coming from behind the dash. My local mercedes dealer says that the noise usually indicates low air con coolant. They have checked the system and say that the level and pressure is ok. When I got the car the air con was not working and the garage (main Merc dealer) said that the expansion valve was leaking, they replaced it and re filled/pressurisd the system.

Any ideas what the problem might be?