Whistling noise from toshiba void unit?

[tristanspence]

Hi. After some pointers as I have looked at a noisy unit this morning. The customer told me it makes a whistling noise a few times thought the night that lasts for 4/5 mins. I have checked the unit on all fans speeds and am pretty sure it's not the fan. Then turned the temp up and down to get the expansion valve to open/close it did make a very faint whistle while opening at one point. The system is a toshiba vrv MMY-MAP0801T8-E indoor unit is MMD-AP0071SPH Any thought on what would cause the expansion valve to whistle intermittently?

[The Viking]

Lack of refrigerant.

The clue is in the fact that it happens at night and I assume it is a recent thing, now that we started to get colder nights.

Happy hunting.

[marc5180]

Transient instability of the refrigerant?
If it's a hissing sound it could be pressure equalizing between the high and low pressure sides of the system across the expansion device when the compressor shuts down.

If it occurs as the compressor is running then as Viking says, the most probable cause is shortage of refrigerant.