The phone's camera has image stabilization which can be either software, hardware (as in mounted on super-fine springs), or a combination of the two. My guess is it's the camera's stabilization lens shaking around in the "yoke" suspended between springs at all 4 horizontal axis. The stabilization lens can also be suspended essentially in mid air between a set of pairs of electromagnetic coils (or a combination of the two), so that again it can jiggle relatively freely within a range allowing the shaking of the phone to be directly offset by the opposite shaking of the stabilization lens. This method is also used by CD players (in combination with RAM buffers), to stabilize the laser and detector assembly in order to be able to continue playing a CD while being shaken such as while jogging.
See below;
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To confirm, if the rattle stops after a couple seconds from when shaken or placed on the table, and then again if shaken - it's the camera - that's my bet.