Groaning Air Con, Engine Off

[Avalanche]

I've had an E39 528 for years and have never had a problem with it but over the last couple of months, there's been a groaning, whirring sound coming from the aircon when the engine is switched off. When you switch then engine and the aircon/heating on, the sound disappears and only comes on half-a-minute or so after switching the car off but when you want silence, it's incredibly annoying.

The sound is coming from behind the aircon controls and when it starts, it sounds as though a motor begins whirring as the sound builds up from a slow start before reaching its loudest point and then remains constant.

The noise stops when you disconnect the battery or take out fuse number nine so it looks like something electrical - a fan, motor or something else - is making the noise but what comes on in the aircon in the car when the engine is switched off? It's always happening as I've heard it on unlocking the car first thing in the morning but after a good search, can't find anything on this problem. I'm guessing that taking this to a BMW dealer would result in a huge bill as they search for the problem so any advice would be much appreciated...

[d3ity]

AC compressor, Heater core, Check your fan clutch as well, Spin the fan while the car is off by hand, if its freewheeling, bad fan clutch. If its difficult to move, fan clutch... should spin with a little resistance.

Check belts too!

[Avalanche]

d3ity, thanks for the reply, just got to get at it now...

[billB]

Avalanche:
If the sound is coming from the dash behind the AC control unit (as u described) it can not be AC compressor or the fan clutch these guys live in the front section of the engine and are belt driven; again the engine works for these guys in order for them to turn; let alone making the noise. Heater cores? the symptom usually manifest itself in antifreeze on the rug (large leak) or very foggy/oily winshield +odor as a result of leaky heating core.

The only thing comes to mind is your heater/AC blower; it does live behind the dash and closer to the firewall; so try to focus on that one for a start.