pit·ter-pat·ter 
(pĭtər-pătər)
(pĭtər-pătər)n.
A rapid series of light tapping sounds.
intr.v. pit·ter-pat·tered, pit·ter-pat·ter·ing, pit·ter-pat·ters
To make or move with a series of light tapping sounds: rain pitter-pattering on the roof.
[Imitative.]