Flash and bang at construction site.?

There is some construction going on nearby my work. i can see it out my office window. I keep seeing these flashes in the sky, directly over the site, followed by a 3-4 second delay and a bang. It's kind of like fireworks, but there isn't the streaming smoke and sparks going up and a bight colorful blast. It seems like it has something to do with the construction going on. Just wondering if anyone knows what it could be.