W211 Cabin Air Filter Replacement and Blower Motor Noise Troubleshooting and Lubrication - On Demand Video Manual
Why do these go hand-in-hand? Well if you do not change the cabin air filter(s) it is likely the dust will work its way into the blower motor and cause squeaking and chirping at low fan speeds. If that noise is ignored you will end up replacement the heater blower motor.