The motors very rarely fail, the common problem is the plastic gear sets; plastic links and the plastic housing. The way you describe the noise suggests to me the gear set is slipping; this is very common and well known on w203.
The factory repair is to remove dash assembly and replace all gear sets and links, depending on how bad, sometimes even the heater box as well, as the gear sets can elongate the housing.
If you can't get any goodwill, I would suggest getting a fixed quote on having all gear sets/links replaced.