Heartbeat sound (3-1-3)
(Other definitions for pitapat that I've seen before include "Light tapping sound" , "quick beats" , "Sound of rain on a window pane?" , "Sound of quick, light steps" .)
(Other definitions for pit-a-pat that I've seen before include "Sound of quick, light steps" , "Light tapping sound" , "Sound of rain on a window pane?" .)