I was just on the 151 Sheridan bus at about Sheridan & Ainslie (I'm not totally sure of the intersection) when we heard a shot fired very nearby. Only one guy got down immediately - I had to hear the second shot before I my non-native-Chicago brain processed what was happening ("... fireworks?").
Everyone got down on the floor until the bus started moving (and a few stayed down a bit longer) - I counted four shots in total, but it might have been five, and there was an echo. I tried to take it like a true Chicagoan, getting right up when the shots stopped and laughing as the bus driver asked if anyone needed to get off at that stop. A gentleman across the aisle from me said, "What was that, a firecracker?" "No," I said.Absolutely disgusting. If anyone has any further information, please post it in the comments.