Owls can freak you out in the woods. They just don't hoot.
As for the knocks, from my experience I think it's a combination hand clapping and smacking a tree with something. Not at the same time, but one or the other. I've heard them do both on recorded audio. The sound signatures are very apparent. I know some people will say they might use rocks to smack against each other for knocks. Our tests have shown this simply isn't possible for several reasons as a way to communicate with others. Since no sticks/clubs is ever found I have no clue what they use, but they use something heavy. I also think they only tree knock for only one or two reasons, mainly herding animals during a group hunt and maybe telling the rest of the group there's intruders in the area. This only applies to a group situation, as a single Bigfoot has absolutely no reason to tree knock. I've put the notion out of my head that tree knocks are used for mating calls and courtship because it helps ease my mind when in the woods, as I have very long hair and even with a beard I do not want one to think I'm a bushy little female. There is nothing that sounds like a real tree knock, not woodpeckers, not acorns or pinecones falling, to confuse these sounds means someone is not properly paying attention to the audio or not yet able to properly identify audio sounds.