Water rocking back and forth inside the toilet bowl is typically caused by passing atmospheric pressure fluctuations in the plumbing vent stack, minor seismic shifts, or someone walking heavily across an uneven wooden subfloor near the bathroom. If the toilet bowl itself is loose and rocking on the floor, any mechanical vibrations in the room will cause the water surface to oscillate. Verification: Check that the toilet base is securely shimmed and bolted down to stop unwanted motion.
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