Ever since the incident I've kept a mike open and recording in one of my production studios. It's very sensitive and even records my footsteps in the hall but so far nothing.
Meanwhile here is the response I got from Demetrius Ivory who does a our weather.
Good morning!
Did you see bright lights also? Lol
I checked the radar at that hour and it was all snow. That means that you may have heard thunder snow. If so it was elevated because it didn't show on lightning indicators.
There is one indicator that there may have been enough lift to create lightning.... Show showers were just developing at that hour. That means that the highest motion or energy was in the area at that time.
Demetrius
I did like the response about from another FReeper who heard it around the same time I did. He seemed to think it might have been a (just) subsonic B1B.
The only problem I have with that is that when the traffic copter hovers near our building the noise is coming from above while what I heard seemed to be in my building or coming originating from below me.
I'll keep the recording going and if I hear it again I'll keep you posted!
Thanks for the update!