Humming from engine when air conditioner comes on in 2002 Audi A4?

When the a/c comes on, there is a loud humming from the front passenger side of the car until the car is cooled. Anyone know what may cause it?Humming from engine when air conditioner comes on in 2002 Audi A4?Yes. The A/C compressor turns on when the climate control sensor reads that cold air is needed to cool the cabin. The A/C compressor is located on the passenger side near the wheel well. Once the sensor determines the air temp is cool enough the A/C compressor turns off. The loud noise could mean the compressor is on its way out. If the noise does not bother you, live with it until it burns out as the expense to replace this item is great- a new compressor from Audi with labor will run $770 which includes bleeding the system and recharging the gas. But have it looked at by a mechanic first.Humming from engine when air conditioner comes on in 2002 Audi A4?It does not know the words.Humming from engine when air conditioner comes on in 2002 Audi A4?It could be the AC compresor is aging or one of the drive pulleys or a bearing is faulty. It is not so easy to tell what the fault is exactly. I will suggest you visit a competent shop or dealer for help.