Pit-a-pat is a term, often used in literature, that mimics the rapid and rhythmic sound made, for instance, by rain hitting a roof, or a heartbeat, particularly when excited or anxious. It is also used to describe the action of something beating or moving in such a rhythm.

pit-a-pat (third-person singular simple present pit-a-pats, present participle pit-a-patting, simple past and past participle pit-a-patted) To make a series of quick tapping sounds; to palpitate