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Seriously. I was CONVINCED it was coming from dash because banging on the dash would re-create the rattle. The acoustics in the car must be weird because it didn't sound like it was coming from the screen.
Because there was NOTHING loose under the dash, one day I decided to check EVERYTHING so I was banging on the dash with one hand and putting pressure on other things to try to find the source of the noise (steering wheel column trim? rear view mirror trim? rear view mirror? trim behind screen? glovebox? - you get the idea). I finally hit the dash and the second I grabbed the phone mount, the noise stopped.
The phone mount in question (magsafe mount made by Jowua), has a foldable telescoping arm. For some reason, that arm came slightly loose and would vibrate/resonate under the right conditions. I had to take the mount apart to tighten some hidden screws around the telecoping arm and that did the trick.