An F/A-18 (I think--definitely a modern fighter) just banked over the Detroit River, low and moving fast. Northbound.
Yep, apparently I still have 9/11 reflexes.
[Update, 12:40pm: I've heard it twice, in the distance. Sounds like it gets close, then banks away.]
[Update II: We have an explanation--it was an F-18, and it was a flyover for the Gold Cup powerboat races this weekend.]