MacBook Pro 16 Late 2019 right side palm rest area has some flex and make a sound during temperature change. Why is this?

After some serious computation or when I place the MacBook Pro 16 down on a table, the right side palm rest area would flex a tiny bit and make a low pop sound. This is not from the speakers, I think as audio is not playing at the time.

I try touching said area afterwards but cannot get the pop, feel a flex or hear a sound. Again, this seems to happen with temperature change, either from within or the surroundings.

What is underneath the right side palm area that could explain for this pop/sound?