Air Conditioner Auto/Auto Max creates loud humming noise in the radiator fan on 2003 Mercedes-Benz E500

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When turning the air conditioner to auto or auto max the radiator fan makes a loud humming noise as well as the back seat air vent start blowing hot air. The front air works fine.
(2) Answers
Your condenser ( on the radiator ) fan sounds like it's worn out. The hot air in the back seat sounds like the A/C is low and needs a charge.
Had the problem with hot air blowing into the rear compartment from the vent in the console. Console was so warm, it was how we noticed the problem. Local Mercedes dealer replaced the control valve for the rear vents. It is a $35 valve that will cost you $400 to have repaired.