This is related to this now closed post that referred to a mysterious beep when aircraft are in mid flight.
I have found out that this happens whenever the aircraft is within 500ft of the assigned autopilot height. Below is the video.
What I don’t know is if this is a glitch or an alert that also occurs in real life and why it would need to do that if it’s the case. Anyone who knows, please let me know. Also not sure if this qualifies as support, so keeping it here for the time being. Thanks!