Tone in the air.

Although this is not a sighting, it is somewhat unusual. This morning, between 1:30am and 3:30am, a very audible mid-range tone was in the air with no identifiable source. I posted it on our local social media group at the time and immediately, others, who were evidently awake, responded saying that they heard it too. I had also called our Cleveland police non-emergency number to report it and strangely the police operator, who said she lived across town from me, had heard to too on the way to her 2:00am shift. No accompanying lights or other indicators. The tone suddenly stopped here at about 3:30am. Although someone in my neighborhood posted that they had heard it beyond 3:30am. Everyone is wondering what the source of that tone was.