Definitely stop shaking it...
Could be a number of things rattling around in your movement (my guess a screw?), but regardless what it is - it could be or is currently causing damage.
Take it to a jeweler or get the back popped off yourself to see what’s going on. I believe your watch has a single use gasket, so it will no longer be water proof when you put it back together, but that’s likely better than a lose movement screw playing a game of ping-pong. If you don’t know when it was serviced last or it hasn’t been for a while, it’s probably best to get it done.
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