Weird Sense of Something Shifting Within iphone 13 Pro

Hello, I posted this yesterday in 'Using iphone', but since I've had no replies I thought that maybe I should post in this discussion. When I tilt the face downward, I sense/feel a movement within the iphone. It made me think something was loose. It only happens occasionally/I can't purposefully recreate it/and it only happens once - I've tried shaking my iphone, but nothing happens then. Does anyone have any idea what is causing this? Should I call Apple or go to an Apple Store? tia