I just found out what that beeping noise in all those 9/11 videos was

I’ve seen a lot of 9/11 videos and I’ve always wondered what that beeping noise in the background of some of them are. Today I found out it is the firefighters’ PASS device giving off its alarm to indicate that a firefighter has not moved in a certain amount of time (in the case of 9/11 videos, it is presumed that there are firefighters who have perished).

Sometimes when I see firefighters responding to an event in NYC I hear this beeping noise too despite no immediate danger and I think it’s because the timer is initiated when a mask with oxygen is put on or something. Not 100% sure about that, but it seems to be the case in the video below. Kind of makes all those 9/11 videos really different now in my head.