Most evenings I keep hearing loud booms that sometimes shake the ground. One time I was in Target on Azusa Ave and heard and felt a "sonic" boom. I thought we had an earthquake or a building fell down. They happen occasionally and only once an evening. This last one was 1/21/18 at about 6p but sometimes I hear them later in the evening around 9p. Further, they have been happening with more frequency since summer 2017. At first I was sure it must be related to the 4th of July celebrations, but now they still happen and are not traditional fireworks sounds. The Nextdoor.com community that I am a part of is baffled, but there are thoughts that it is coming from a "speedway". What speedway? What are they doing there? What is this concerning noise?