Humming noise in cabin when AC on

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Car is an Audi A3 8p1 2005

Am trying to find the source of this humming noise.
Humming can also be felt as a vibration on the dash ans steering wheel.
Kinda sounds like the noise water running through old pipes make in a house.

• Happens when the Air Con has been running for a while.
• Goes away when I switch the Econ on which turns the Air Con Compressor off.
• Noise not present in the engine bay when this is happening and could not find a hint of the noise anywhere there.
• No squeaks for from the belt on unduly noise from the compressor or compressor clutch.
• No VAGCOM fault codes from the HVAC system.

Have a video of it happening:
https://dl.dropboxusercontent.com/u/95805075/A3HVACHummingNoise.mp4

I have ruled out the blower motor as it goes away when the AC is off.

Could it be caused by the condenser vibrating\pipework as the Aircon gas flows through it?

I'll try to inspect the pipework next time its happening but for now, any ideas are appreciated.

 

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Had Mark from http://www.ac4cars.co.uk/ come to take a look.
Great guy and knows his stuff.

Vibration seems to be coming from expansion valve \ evaporator area.
May be vibration due to not enough gas. Also my condenser fan does not spin, big radiator fan is ok.

Going to get the fan replaced and a re-gas done so hope this solves it.

 

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Finally had the fan replaced and the system re-gassed.
Seems to have solved the issue!

Mark @ http://www.ac4cars.co.uk/ - highly recommend!