1998 Other Mercedes Benz Models

Mercedes SLK 230 One owner kept in garage, very little driving, short trips. Problem, We are getting an intermittent, high pitched sound coming from the middle of the dash board. It lasts about five seconds, then it goes off again every thirty seconds. It does this when the air conditioner is on or off, also when the heater is on or off, however if we hit the EC button, with the system on, it stops the sound. There is nothing in the manual. Any ideas where to start to solve this problem. (EC economy Mode)

Thank you