Just a guess...........could it be something to do with the air-con system? VW Passats (98-05) suffer with the faliure of the motor that closes the flap when the blowers are set to re-circ, and this manifests itself with a constant tick-tick-tick sound as the motor tries to work but is failing.

Without spending any money, I think the best next step may be to find a mercedes forum, and post your question on there, in the relevant section. Hopefully you'll find someone with more knowledge on there, as my guess could be way off the mark.

Many thanks for these further replies. I'll have to get it into a garage then.

Sorry if this sounds obvious but is the car ok to drive until then?

It appears to have dropped into LHM normal problem is the EGR valve could be the solonoid of this valve you can hear,also they have two batteries a small battery is situated under the plastic cover under the bonnet on the left hand side this is used for many things including running the heater and ac when everything is switched off there is push button on the dashboard for called rest heat which activates this program.