Periodic mysterious (industrial ?) sound inside the loop

I live in the 77019 area of Houston. There is a low pitched sound that occurs approximately every 5 minutes and lasts for approximately 30 seconds. I just can’t determine what it could be. It’s a low-pitched drone – sounds industrial. From my balcony, it seems to be coming from northeast of my location. Perhaps the Heights area. It’s not particularly loud at my location, but it seems that it would be loud at its source. Any ideas?