I have a 1999 SLK230 Kompressor with 90K miles. I am getting a very annoying high pitched 1000 HZ noise coming from under the glove box. It happens only when the engine is running and the fan switch is turned off. It comes and goes but will stop if I turn the fan switch on to any speed. 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. The noise would not stop. It may even 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?