Not a milestone per se, but did your LO ever go through a phase where they were afraid of loud noises? How old are they when the fear started? How old were they when they overcame the fear?
My 9 month old can't handle me vacuuming or turning on the garbage disposer unless someone else is holding him. Makes certain chores tricky at times! I remember my oldest going through the same thing, but mommy brain made me forget when it started and how long it lasted.