The heater blower motor can become abnormally noisy. Replacement of the blower motor is commonly necessary to correct this concern.
Car Problem Reports
Mercedes-Benz ML500 Heater Blower Motor May Become Noisy
Mercedes-Benz ML500 Problem
If i run the air or heat in my car it will start to get very loud, especially when idle. The MB dealership that I had it serviced at when I got the car didn't say anything was wrong with it, so I assumed it was part of the package. That was at about 110k miles. 6mo later 113k my vents dont work unless on the highest setting. The heat takes forever to warm up, and defrosting doesn't happen in less than 20 mins.