Annoying "eeeee" sound from under glove box
 

I have a 1999 SLK230 Kompressor with 90K miles. I am getting a very annoying high pitched 1000 HZ noise (say "eeeee...") coming from under the glove box. It happens only when the engine is running and the fan switch is turned to 0 (off). It comes and goes but will stop if I turn the fan switch ON to any speed. It does not change pitch and is not mechanical like a fan.

I removed the kick panel under the glove box and took off all the cables to the fan and other connectors I found and even removed the air filters which were clean. The noise would not stop. I played with all the switches, lights, radio, etc. and nothing changes. It may be coming from above the fan but it is hard to determine lying upside down with a flashlight.

Any idea what else is under there or what can be causing the noise?

Noise
 

Sounds like a vacuum leak coming from a leaking valve that operates the flaps. Could also be the vacuum switchover block. Those things crack and makes lots of noise all the time!! Just like you described. Good luck