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 ?