Hi all. I have exactly the same problem. Just recently I bought a Pro 16" and when I shook it up I found the same problem. And exactly the same, if I close the Mac and lightly press my fingers on the sides of the lid and shake it, the sound goes away I decided to go straight to Apple And ask them. Turns out they are aware of this problem. They said previous models (2019) had the same problem. They also said that this will not affect the functionality of the device, and that there is unfortunately no solution to this problem. They only offered to replace the screen, but said in advance that they do not guarantee that the strange sounds will stop... Interestingly, in the Apple store where I went there are the same Macs (14 and 16) with the same problem.