Strange whining noise when AC is off

[skmattwell]

Started up my zed after a month of it not being in use and noticed a strange new noise once it warmed up. If the aircon is off there is a strange whining/hissing noise from the engine bay that fluctuates (as demonstrated in the video below).

Turning on the ac makes the noise completely go away. Any ideas what this is?

Sound with AC on:

Sound with AC off (a few seconds in you can hear one of the pulses I was talking about):

 

[chanlon1]

Funny you should post this as I think I heard the same in my e86 today. I regassed the AC system last night as I was fairly sure it had a leak. The system survived the vacuum test that was done and so was regassed successfully. I did notice the pressure did drop slightly over a 30 min window.
Today I tried the AC and it seemed to be colder than before. When the car was stationary and idling, with the AC button pressed there wasn’t any noticeable noise. But when I turned the AC off, a strange noise like yours sounded. No idea yet why.

 

[smorris_12]

Listen to see if the noise cycles with the compressor. When the compressor clutch is released then the belt is running on the clutch bearing (which is when it can make a noise.) With the clutch engaged the bearing is locked and doesn't move.

It's the same reason release bearings only make a noise when you declutch the engine.

 

[Pondy]

Listen to see if the noise cycles with the compressor. When the compressor clutch is released then the belt is running on the clutch bearing (which is when it can make a noise.) With the clutch engaged the bearing is locked and doesn't move.

It's the same reason release bearings only make a noise when you declutch the engine.

This.
Chances are your a/c clutch/ spindle bearing has corroded slightly with lack of use.
The electromagnetic clutch is 'loose' when not engaged (ie when the a/c is not running) and just spinning via the drive belt. Once under load (ie pulled in and running) the clutch is 'solid'.

A liberal dose of WD40 (or similar) on the outer clutch plate and spindle may quieten it down. Just try and avoid the drive belt, or it could squeal.