What causes the creaking when putting pressure near camera or SIM slot

So for some Pixel 4a devices if you apply slight pressure towards the back of the phone (most common back of the phone just below the camera or between the power and volume buttons), there will be a creaking sorta sound. Does anyone know what causes this?

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I had the same issue and now a crack has developed on the corner leading from the edge of the screen to the camera.

There are multiple reddit threads on this issue as well.

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