We have a 2000 SLK-230. Just recently the car has developed a very soft, high-pitched "whining" noise that occurs intermittently. It appears to be coming from the passenger side of the vehicle and will occur when the engine is running, regardless of whether the car is moving or not. Car has 53,000 miles on it and runs perfectly. This noise just started happening a few days ago and occurs every day (car is driven about 10 miles per day to commute to work).

Any suggestions?