Not exactly in Detroit proper, and hardly the most burning question, but...

I have occasion to be in Dearborn every now and then at the noon hour. Often, I will hear a deep whistle sounding at the stroke of 12:00. I thought it might be the Rouge Complex signalling for lunch or somesuch, but that seems a little far away. I'm usually in the area of Michigan Ave and Outer Drive when I hear it. Does the City of Dearborn have a "noon whistle" I don't know about? I've searched the internet pretty thoroughly on the topic to no avail. I'm certain it isn't the steam locomotive whistles from Greenfield Village.

So...what the heck am I hearing?