Disco 2 Air con whining

When engages sometimes the air conditioning compressor makes a whining noise, does anyone know what causes it? (noise sounds a bit like a supercharger) and it goes at revs.

I am guesses compressor last legs, but could it be pressure levels or something else?

All i can tell is that i doubt that it's about pressure cos if the pressure is not within accepted limits the compressor is disabled by the dual pressure switch

I accept Fery's comment about excessively low pressure, but on another Denso compressor based vehicle I have, the compressor makes a slight whining noise at engine idle when the gas pressure is reduced, but not enough to cause it not to work. The whine isn't noticable due to road & engine noise at higher engine / road speeds.

The system has had a slight leak for more than 5 years, so it's happened 4 or 5 times over the last few years & it appears consistent every time.
Every time I notice the noise, I check the AC low pressure side with the guage on the top up cannister & it reads on the edge of "green", somewhere around 30~35 psi. A minor top-up from the cannister to approx 40 psi & the noise disappears & the AC appears slightly more effective for another 6 months plus.
I've no idea why it should whine at lower pressure - as I assume the compressor is probably working less ?

OK I wil check pressure again later thanks

Slightly high, going to work tomorrow will check on arrival and release a little if needed, was high 30s last weekend, around 50 now

Don't go mad releasing pressure just yet.
I think the pressure goes up a lot with increased ambient temperature !
I'm not sure what it is where you are, but I saw 34C on the car display much earlier this morning - alot higher than the probable 24~25C when I measure the pressure last week.

38 on the display!!!!