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.