Mercedes 320CLK - AC/Heater/Dash noises

My wife loves her Mercedes 1999 320CLK. Low mileage (120,000), excellent interior and body condition. One problem. When she turns on the heater there is a very high pitched whistle that gets loud and then slowly tapers off. This repeats from time to time. Randomly there is a sound like a flap/door slamming shut. All sound comes from behind the dash. Also, from the dash behind the steering wheel area you hear a sound kind of like someone squeezing a plastic water bottle - plastic crackling sound. Our mechanic took a look at this and told us that some sort of hose was loose. It was reconnected and things got better for a little while…now its back. Any ideas? Once in a while this happens with the AC.

It sounds like more than one problem. The door slapping is probably the air blend door. This blends heated and outside (or recirculated) air to adjust the cabin air temperature to your setting. Did your mechanic look at that?

No he didn’t. That problem is intermittent and it didn’t come up when he tested the car. The other problem did (high pitched noise). The sound of a plastic bottle being squeezed is also intermittent. Thanks.

Are there any water sounds associated with the sound in the dash? If so, it might be a blockage in the evaporator drain. As the noises become more frequent, a mechanic can identify them. I know it’s hard for me to patiently wait for intermittent issue to become constant, but a little patience will eventually pay off.


The various doors in your plenum are vacuum operated. When the rubber diaphragms in the actuators fail, the result is often whistling type sounds

+1 to db’s post. That was my first thought too.