CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Whistling Mercedes

I own a 1997 Mercedes C230. This morning, it started making a funny sound when I started it. The sound is apparently coming from the AC vents in the center of the dash since, when the air conditioner is turned off, the sound goes away. The sound occurs in all gears, stopped, driving. It sounds more electronic than mechanical. Here’s what happens: first there are two brief chirps–tweet, tweet–followed by a high-pitched e-e-e-e. Not very loud.
The e-e-e-e lasts for about three seconds, then stops. Five or six seconds later, the pattern repeats: tweet, tweet, e-e-e-e. Six seconds later, it repeats.
And so on, for as long as the car is running. Any ideas?