How hot is the room they are in?
What does the thermometer say?
If they have a heat plate, the rest of the brooder doesn't need to be hot at all, it can be quite cool/cold.
They are inside the coop, so I assume no drafts right?
Could be distress calling because you leave or they hear the storm and it's scary.
Only thing I see that I might try is grinding their feed up smaller for a few days. The chicks are tiny and sometimes crumbles can be too big. I throw...