Does it happen more when it is windy outside? Does the toilet in question have the UniFit adapter? Air pressure in the vents can cause the water to rock in the bowl...some can slosh over the weir and down the drain. Also, after a fill, it may take it a few minutes to equalize the water level. This is more noticeable on the older version of the Unifit (the newer ones came with some sound insulation on them). If you shut the water supply off, does the noise stop?