whir

whir (http://definr.com/whir)

     n : sound of something in rapid motion: "whir of a bird's
         wings"; "the whir of the propellers" [syn: whirr, whirring,
          birr]
     v : make a soft swishing sound, as  of a motor working or wings
         flapping [syn: whizz, whiz, whirr, birr, purr]