After roll call a group of police officers were standing in the parking lot talking. A very loud, low rumble, like a freight train is how we described it, passed overhead sort of slow. We initially heard it when directly overhead and was so loud it drowned out our conversation. We could hear it moving away from us for several seconds then it was gone. It was dark outside and couldn't see a shape or size. Like nothing any of us have ever heard before.((NUFORC Note: We have amended the date above, to reflect a sighting on March 16, 2016. PD))