My one experience with tree knocks that I can assign any meaning was my encounter with two adult and one juvenile BF. They were traveling up a creek, one on each side of the creek. The first I became aware of them was a whoop on one side of the creek answered by a similar whoop from the other side. My immediate reaction was that it was two owls whooping back and forth to each other. Then the azimuth of the whoops began to change, indicating they were moving, and giving the impression they were flying up the creek. My thoughts to this point was that this was really strange owl behavior. I dug out and thought I turned on my audio recorder, setting it on a post. I stood and just waited for the two owls to come flying up the creek past me. They continued to whoop back and forth to each other as they progressed towards me. Then I heard the brush crashing and heavy footsteps on my side of the creek headed right towards me. Obviously not owls flying. This crashing and footstep thuds were loud and very similar to the T Rex night encounter portrayed in the original Jurassic Park movie. Something large and very heavy crashing through the brush. I stepped off the trail, attempting to hide, and waited for what I now suspected was a BF headed right towards me. Suddenly I heard and felt a huge thud in my feet as the BF apparently saw or smelled me, and went into a crouch just a few yards away in the brush. 15 seconds after the thud, there were 4 rapid tree knocks. That was the last thing I heard from either. It seems fairly obvious to me that the knocks were to warn the other BF on the other side of the creek, that I was present. The brush at that location would prevent them from seeing each other. Those knocks had to have been a warning to the second BF, because the one that made them had to have known I known I was there, and the second BF did not know I was present. For those that do not recall, or have not heard about my encounter, I did not see the nearby adult but got a picture of the juvenile peeking at me over some sword ferns, before they both silently withdrew.
Other knocks I have heard seemed to be a response to me slamming my truck door when I parked it near an active area. These were single knocks that happened twice in response to my slamming truck door. Perhaps some kind of response to my door slam. EIther in response to an assumed knock or warning to other BF in the area, who may not have heard the door slam, that I was present.
I can easily assume from my experience in this context that knocks are a warning, not at me, but for other BF in the area. Other knocks in other context may have different meaning. If I am correct, those that think making knocks will attract BF, might actually be warning them that you are present so other BF can avoid you. I also do not make knocks for that reason.