whirr

(noun) sound of something in rapid motion. Synonyms: whir, whirring, birr. Examples: "Whir of a bird's wings." "The whir of the propellers."

(verb) make a soft swishing sound. Synonyms: whizz, whiz, whir, birr, purr. Examples: "The motor whirred." "The car engine purred."

(verb) make a vibrant sound, as of some birds. Synonyms: churr.