Flock of fast birds with white/grey undersides

Over the past week or so, I’ve seen a flock of birds race by several times. They fly so fast that I can’t even identify them. So I was hoping that someone else would know what I was on about. 

These birds fly in a flock of around of 7-10, pretty low down (probably just above house height) and have dusky white/grey undersides. They fly very fast almost racing each other. Also, they didn’t look duck shaped but more similar sized to pigeons