What I found is that written evidence for knocking on wood (and for the history of gesture as a whole) is remarkably sparse. In the Penguin Guide to Superstitions, folklorist Steve Roud offers a skeptical take on the provenance of knocking on wood due to the near total absence of written records about it:
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EDF and Natural England agree that without the extra acoustic deterrent, around 2.6 million fish could be sucked into the cooling pipes every year.