Hi,

I had a Fujitsu multi zone unit installed this past spring. There is an indoor head for the upstairs unit, and an air handler for the lower level, which uses the HVAC system.

I live in Minnesota and temps drop pretty low in winter. During nighttime, the home temperature drops from 70F to 66F, where it is maintained at for the entire night.

On nights where the temperature drops into the 20s, I have found the heat pump to make squealing noises when it starts up. It starts out with a scream, and then throughout the night, it will kind of sound like a rusty water valve opening and shutting.

The noise does Not happen if the air handler or shut off and the upstairs head is the only unit running. It Only happens if I have the downstairs thermostat set to use both heat pump and boiler. The installer does not know what this would be, because it is also difficult to recreate the right conditions for the noise to come on. I am assuming it occurs as the unit goes into defrost, but it is difficult to tell.

Here are the model numbers for the units I have:

Outdoor unit: AOU24RLXFZH
Upstairs head: ASUH12LPAS1
Ducted unit: ARU12RLF

Because the noise occurs in the middle of the night, it has been difficult to capture it properly, but I was able to record an instance of the noise:
https://drive.google.com/file/d/131EdOerB1vgXA38Fja5nPyLR6180AuGm/view?usp=drivesdk

Hopefully the google drive link shows up fine. I would greatly appreciate any insight anyone might have.