I had three Mercedes so far and each of them suffered at least once from the problem of the squeaking or chirping blower motor during my possession time.
The noise changes with the speed of the ventilation and tends to be more audible in the lower to medium blower stages. It can also occur temporarily, e.g. during braking or cornering, and is sometimes temperature-dependent.
The permanent solution would of course be to change the fan with a new part e.g this one.
So far I have had good experiences with the re-greasing of the shaft and can at least recommend you to try it before you replace the component.
I first lubricated the first fan in the w209 (CLK) with WD40, but this didn’t hold 5´000km before the chirping noise could be heard again. Afterwards I re-greased with thicker multi-purpose grease, which then kept more 40´000km up for sale.
In the E-Class (w211) the noise didn’t occur after greasing until the sale to 10´000km anymore and in the C-Class (w204) I hope now for similarly long durations.
On my three models, the fan was located behind the passenger’s glove compartment and can be reached in the footwell after removing the cover. For my CLK and the E-Class, I removed the fan motor for lubrication, which unfortunately didn’t work for the C-Class. Here I only lubricated the lower bearing directly through the small lower opening in the installed condition, which fortunately also helped.