Grumbling air

FIRST check the drive belt to make sure it's in good condition and tensioned correctly. Then check the compressor. The refrigerant gas carries a lubricant which is used to lubricate the compressor. If the system runs low on gas you can damage the compressor, so make sure the system is correctly charged with gas. If it is low, there is probably a leak in the system that will need checking by an airconditioning expert. Your car is 12 years old, so it's possible the noise is being caused by worn bearings in the compressor.