The heater blower motor can become abnormally noisy. Replacement of the blower motor is commonly necessary to correct this concern.Google+
Car Problem Reports
Mercedes-Benz ML320 Heater Blower Motor May Become Noisy
Mercedes-Benz ML320 Problem
Average Mileage: 137,833 mi (63,000 mi - 235,000 mi)
Visitor, 2002 Mercedes-Benz ML320
The back seat blower remained on all of the time and was extremely loud. My mechanic had to unhook it in order for it to stop running the battery down.
Visitor, 2000 Mercedes-Benz ML320
i have no air or heat...just air coming out and it's loud
Visitor, 2000 Mercedes-Benz ML320, 102,000 mi
AC fan became noisy
heater (fan) only works on 1, and the 2,3,4 make a lot of noise and vibrate so hard, but no air
Visitor, 2000 Mercedes-Benz ML320, 100,000 mi
Ny blower motor is very noisy
The heater blower motor is very loud and now does not blow air through the system.