I have heard similar noises but there is a ready source for that sort of noise in my area aside from Walnuts. We have lots of granite mines in the area. Blasting underground produces this sort of a sound and sometimes a light but noticeable shock wave.
I’ve heard that kind of noise in the woods not far from a quarry in dekalb county. This was a rumble under foot. Like giant rocks rubbing together. I mini earth quake if you will. Don’t recall hearing a bag of flower dropped from a tree though