My '96 E300D makes high pitch sound when I turn on the climate control. sometimes it's won't stop. sometimes it's make that sound intermittenly. When run the defrost most it stop. If climate control is off, then it is quiet. Anyone know what cause it to make this aweful sound. I have to turn on the radio loud to drown it.
Thank you for any help or tips you could give.

If you turn on the a/c and the sound starts then it is probably the BOSCH A/C blower.

This happens when the carbon brush is a bit worn out. A cheap remedy is to apply a few drops of WD-40 to the carbon brush. DON'T apply to much since the motor might not turn due to too much wd40 oil. This would last a few months.

The carbon brush can be replaced DIY but it is quite tricky to do. It would also be quite difficult to find a shop to do this for you. Also, this carbon brush cannot be found on a Benz parts list. If you have the cash, just replace the whole A/C blower to totally fix the problem.

Thanks.