My heart goes pit-a-pat. Throbs, palpitates. “Pat” is a gentle blow (Welsh, ffat), and “pit” is a mere ricochet expletive. We have a vast number of such ricochot words, as “fiddle-faddle,” “harum-scarum,” “ding-dong,” etc.
Anything like the sound of a rat Makes my heart go pit-a-pat.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894