It's common to say "a gentle wind", but is it OK to say "a wind"? I just noticed that there's a novel named "A Wind in the Door", in which case I guess "A" could be used here due to the modifying "in ...
Imagine yourself walking in the woods with children. One child is saying, "there is a big hole in that tree's trunk." You answer, "perhaps a/some bird lives in there." Would you use a or some? ...