Anna’s heart went pitter-patter as she opened the letter.pitter-patterpit‧ter-pat‧ter /ˈpɪtə ˌpætə $ ˈpɪtər ˌpætər/ adverb literary 
Anna’s heart went pitter-patter as she opened the letter.—pitter-patter noun [singular]: the pitter-patter of small feet running up the stairs
WORD ORIGINpitter-patter
Origin:
1400-1500 From the soundCOLLOCATIONS FROM THE ENTRY Anna’s heart went pitter-patter as she opened the letter.
with quick light beats or steps: Anna’s heart went pitter-patter as she opened the letter. Anna’s heart went pitter-patter as she opened the letter.—pitter-patter noun [singular]: the pitter-patter of small feet running up the stairs