I think it might be trains starting to move. Here's why -
I found myself on the Berwyn Marta platform the other day when a 50 car freight train stopped in front of me. When it started again, the noise was incredible. It was one loud, deafening "boom" that echoed off nearby buildings.
Given that the 18th "Air Line" runs right through the Sloop and that I see freight stopped on that track many evenings, I think this might be the source of at least some of the booms we hear.
I don't think that is it. I have lived in the neighborhood now for close to 5 years and it is only within the past few months that these "booms" have been sporadically happening. If it were trains, I think it would have been consistent over the past few years. We had one this Saturday (10/20) around 10:30 PM that scared the crap out of me. It sounded like it was close (I live at about 14th and Michigan).
I heard the boom on 10/20 and also live around 14 and michigan. It sounded and felt like a bomb went off. It felt much closer than the one a few weeks ago. It definitely has nothing to do with the trains.
It's not the train. Someone is either using powerful fireworks or even packing their own (using gun powder and, say, a tube - I did so as a teen). This last time, my mother saw the flash and we both spotted a large amount of smoke coming from the big empty lot (the former development site with chain-link fences) on the 1300 S Block of Wabash. It was quite a bit and took a while to dissipate. So, either someone it throwing the device over the fence or from the roof of the adjoining building. I've suspected that the explosions were occurring there, but this is the first time I've seen the smoke.
Please call the alderman, as per my previously mentioned 'experience', I can tell these are large explosives and someone is going to get hurt, either by flying debris or a dud that is discovered by someone else.
I also live at 14th and Mich and the boom heard on 10/20. It happened in the alley between Michigan and Wabash, between 13th and 14th. A large cloud of smoke could be seen coming from there. After seeing the cloud and thinking back to childhood, I suspect the booms are kids setting off "h-bombs" (basically a 2 liter, acid and aluminum foil). If we investigated a bit, I believe we would see the evidence in the alley. I don't know how the stop it, as it happens different days and at different times - also the kids (or immature adults) would be long gone by the time the police got there.
I also totally heard the boom on Saturday and I thought immediately of the postings here. I live at 15th and wabash right by the freight train tracks. I immediately went to my window...no trains in sight. I know exactly what JeffWagg is talking about with the trains shifting and the booms that come with it, and can tell you that what I heard on Saturday was not coming from the train line by me. I literally live in the building that is up against the train tracks, so I am very familiar with the train shifting booms. Although very loud and impressive, this was something different. I also thought immediately that it sounded like a bomb or an explosion, but didn't see any smoke or light. Maybe I am too far south. Next time I hear that noise I am going to call the police. Crazy!
I can report that I saw and heard two booms in the past week:
1. The week of 10/14, I forget which day, I heard a boom about 11PM. I looked out the window and saw smoke billowing from the empty lot on Wabash, east side of the street, and three teenagers running away. Clearly a firework, probably an M-80.
2. Yesterday (10/22) at 10:15AM, a transformer exploded at Congress and Dearborn. ComEd was already there, they probably inadvertently set it off. It was extremely loud and bright.
I know for a fact that the one a couple weeks ago was a transformer. We saw everything from our window, the flash, smoke, police, and fire truck. A day or two after there were rubber blankets on he high lines so someone was probably working on it or the blankets were there to protect the damage. (This was at 13th and Michigan in the alley behind A'Cappella). As a matter of fact, it looks like the (orange) blankets are still there. I've heard that sometimes animals get between the phases or phase to ground causing them to blow up. Not sure if there's any truth to that but not a good visual that's for sure. If every boom is a transformer, I don't know. I find it hard to believe that there are that many transformer blow ups in the South Loop. Maybe its a mixture of things. Hope that helps your concern a little. BTW I'm an electrician so I'm a pretty good source. :)
I heard one of the booms a few weeks ago. I live at 9th and Wabash. It's definitely not a soda bottle or a popping squirrel that got caught in the wrong place, particularly if it occurred in the general area everyone's referring to. When I heard it, the first thought I had was that it sounded like artillery.
I think SL Resident (and others) are closest to the source. It's probably some kind of simple but moderately high explosive, and if it keeps happening, someone is going to be seriously hurt or killed.
If it is happening at the vacant lot, I hope the police can go out there to look for evidence. If they find something, I don't think it would be difficult for them to put out a few fliers for all of us to be on the lookout in that area.