I reached out the Bambu to confirm what to do and this was their response:
Judging from your video, the noise could be from the friction of the Z belt and wheels. The four areas highlighted in red circles in the diagram are potential sources of friction.
We recommend applying lubricating grease at the Z belt tensioner idler wheel to address this issue and make the noise.
If the noise persists after lubrication, please refer to the wiki: Replacing the Z timing belt | Bambu Lab Wiki
You can follow steps 5 and 6 in this wiki article to remove and inspect the tensioning idler assembly of the Z belt for any misalignment shown in the picture below.
EDIT: They also sent a video to show where to put the grease