As in a bug that flies around and bothers people.
(Also see "fly" as in airplane...)
Because the word "fly" is so short (three letters) this concept is often just spelled out. But, here is a sign that works pretty good for indicating the insect known as a "fly."
If you want to talk about a fly buzzing around, I'd use an "F" handshape. Pretend you are holding a fly between your index finger and thumb. Then wiggle your hand around as if the imaginary fly between your finger and thumb were buzzing around in the air.