A/c noise

[weasley]

I recently noticed a new noise from 'the engine'. An extra whirring noise that is linked to engine speed which tends to be there for a little while after startup but goes away. Today I found that it stops immediately if the A/c button is pushed (to switch it off) and returns immediately when switched back on again. I therefore suspect either the compressor or the clutch. The a/c is still blowing cold when expected. Any thoughts/experience?

[Tech1e]

Doesn't have a clutch but will be under more load when cooling (it never switches off). I'd pop the belt off and double check the noise has gone with no belt, then have a feel of the tensioner, alternator and compressor for anything obvious.

[Ryeman]

Are we talking about it being noticeable when driving at normal speeds or stationary with window down frinstance?.

[weasley]

Noticeable at slow speed (in a car park fr'instance) from inside the car, windows up. Like a noisy bearing or something.

I wonder if the compressor has enough oil?

[Ryeman]

I guess the button activate the fan also which I find is the most noticeable noise in an enclosed space but it should be possible to analyse it with the bonnet up in the open I would have thought.

[weasley]

I'm certain it's not a fan noise, as the correlation between activation and noise is immediate.

There must be some mechanism that enables and disables the a/c, thus changing the load on the compressor.

[James@RRG_Skoda_Rochdale]

The variation of internal capacity flowing through the compressor is what changes the impact of load on the engine, as techie has already said - it never truly turns off, it reverts to 2% activity to keep mechanical components moving and refrigerant flowing.