How can I make a knocking sound like this guy playing the blues?

I was watching a Boom Boom cover: https://youtu.be/WL8wS7D1XbE?t=45

What I really like is that you can hear a cool knock/tap as he plays. Even when he isn't playing a chord. He could be playing the riff on one string it seems like and he's still able to also knock/tap. How is he able to do make a knock even when he's plucking on one string it seems like? Can someone enlighten me?

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