Odd aircon issue

[Grantus]

It is starting to get quite warm here in NZ. Yesterday I used the aircon for the first time in over 6 months on my E34

When I engaged it there was a knocking noise from the front of the (M50) engine bay which disappeared once I disengaged it.

I have checked the electric fan and it is not that.

I have replaced the small polyribbed belt and in the course of investigation tried pressing the switch with the engine running after having removed both belts (very briefly).

The weird thing is that the only rotating thing in the engine bay was the bottom pulley and the noise was still there when the aircon switch was pressed.

I cant figure out what could be causing the noise.

Any ideas anyone?

[bmwrp8]

I had the same issue way back. Check your a/c compressor. I replaced mine

[sal_park]

Yesterday I used the aircon for the first time in over 6 months on my E34
Any ideas anyone?

Yes, if you don't use your aircon regularly it will break :( I know that's not much help now, but once you've got it fixed you need to use it all year round (say at least once a week). It's cold(ish) here in the UK atm, and my aircon is still on.

[Grantus]

Another strange thing is that the aircon is working fine. It is the noise that is the problem.

[Tiger]

Probably the tensioner pulley is bad... take belt off and give it a spin with your hand... if it sounds or feels like a rattle... it is bad.

[KevinGoFast]

Huh, weird.

[Grantus]

All tensioner pulleys are OK, I've had the belts off and checked them. The noise is there with no belts connected. It really is one of those oddball problems. I'm wondering of it is a solenoid pulsing, but it seems to tie in with engine revs.

I wondered about the crankshaft damper setup, but cant understand what relationship the aircon would have with that,especially as the noise occurs when there are no belts connected and the fan is removed.