flap around
To wave about in the air, possibly due to being unencumbered or unrestrained. You better secure that sheet—otherwise, it'll be flapping around in the wind. A: "What's that noise?" B: "Oh, just the flags flapping around."
flap around
[for a sheet of something] to blow, flop, or slap around, perhaps in the wind. The sails flapped around, making a lot of noise. The awning flapped around during the night.