What causes the humming sound when the BMW X5 air conditioning is turned on?

[JamesLynn]

There are different reasons depending on the situation, as detailed below: 1. The sound occurs when the AC is turned on but disappears when the AC is turned off: Worn compressor bearings; Dirty air conditioning pipes or condenser; Defective suction pipe of the mechanical supercharger in the evaporator. 2. The sound persists regardless of whether the AC is on or off, and becomes louder with higher fan speed: The label of the blower has fallen off into the air duct; Worn blower bearings. 3. A humming sound occurs when the air conditioning is on and the car is accelerating at low speed: Low speed indicates weak engine power at this time, and turning on the air conditioning increases the engine load. Accelerating further demands higher power from the engine, resulting in a humming sound, which is mostly due to issues with the engine belt or tensioner pulley. After ruling out problems with air conditioning components such as the compressor, evaporator, blower, etc., check if the belt is aged or loose.