A4 Noise when A/C on

[franweez]

I have a 2002 A4 1.8T with 78,000 miles. Wierd noise coming from behind radio when the air condition is on. I've isolated the problem to the ECONO button. When the ECONO button is not lit, the AC compressor is on and you can hear the humming load, which is real loud. When I press the ECONO button, the red light turns on, and the A/C compressor turns off....the noise dissappears. The mechanic replaced the fan in front of the radiator....costs $600 but that did not fix the problem.

I brought the car back today and now he wants to rip the interior out behind the radio and find the root cause of the noise. Can someone tell me what happens mechanically when the ECONO button is pressed or depressed. Has anyone had this problem before?