There is a high-frequency noise that occurs when the air conditioning is being turned off.

[colinallcars]

Greetings, we just acquired a CX90 vehicle and are now encountering a shrill noise when deactivating the air conditioning system. I will attempt to record a video and share it at the earliest opportunity. However, I would want to enquire if anybody else has encountered this issue and if there is a Technical Service Bulletin (TSB) available to address it. I was quite aback by the presence of this feature, considering it was absent in our pilfered CX90.

[paulh2602601]

Is the sound localised to a certain region? (What is happening behind the scenes?) Which seating arrangement are you referring to: the front row, the middle row, or the back row? Is there just one air vent or are there several air vents?

[colinallcars]

The sound is audible in the vicinity of the car's front, but not within the vehicle.

[colinallcars]

The sound is akin to a "whine," similar to the sound produced by a microphone. Extremely high-pitched.

[paulh2602601]

If there is a persistent noise like a continuous whistling tone, it is possible that a vacuum line is experiencing a leak or a loose seal. I own the inline 6 engine, which is equipped with mild hybrid technology, and I do not perceive any noise while the engine is idling or when the air conditioner is turned off. If you have a plug-in hybrid, I am uncertain.

[colinallcars]

We own a high-output MHEV (Mild Hybrid Electric Vehicle) that exhibits intermittent behaviour. Specifically, after driving for a certain period and thereafter shutting off the air conditioning, the vehicle emits a loud sound that gradually diminishes over time.

[allmightyASTON]

Inspect the pulley located on the front of the air conditioning system, outside the vehicle's hood, and determine whether it emits a high-pitched noise when it comes to a stop.
I haven't examined it to see whether it has its own belt as well. In my opinion, they are the most basic items to delete initially.