Well mine came back a few days ago from the service center. Told that the sound level was normal compared to the other Y's. Also told that it was the heat pump and that it will be normal for it be in use a lot. What I'm not sure of is how often and how long it should be running. In my case, it's literally on all the time. Now I don't know if the sound is amplified or changes when using other components like HVAC vs supercharging given other temperature differences.
I was watching this video series on the octovalve for the heating / cooling. It's a 3 part series.
https://youtu.be/rgmBpEQtJ1s Makes me believe everything is connected --- HVAC, battery management, coolant, etc.
So with this mind, I have no frame of reference whether this is ALWAYS supposed to be on vs on demand for when needed. If this on-demand, then i might speculate there might be some bad sensor possibly and related to this
article (there's a thread on this forum as well but i can't find it) where the three heat pump sensors on 2020/2021 Y's are defective and need to be replaced.
Either way, i'm going to monitor for a while and document usage and times. I was asked on my SC appointment for times and dates when this happened so they can compare to logs and events. After i've driven and enjoyed the car a little longer, i'll have them look and check if my car is impacted by the sensor issue which might be causing my constant humming.