Superstitions – “Knock Wood”
Many people have the minhag to knock gently the table by benching when saying the (individual) Horachamons
I have seen this with elderly people too who seem to be Minhag-conscious and have taken presumably from previous generations
My assumption is that this is against the Ayin Hara and similar to what people say “Keyn Ayin ora”.
After a bit of reserach it however seems the source is gentile and not Torah but – lehavdil – pagan related.
I was wondering what the Rovs opinion is on this and in particular as this seem to be very common in some circles
This custom seems to have to do with superstitions at best and as you mentioned rooted in pagan belief at worst. Hence it should not be practiced by Jews. The fact that people do it is not necessarily proof that it has Jewish or valid roots, as this is not a wide spread custom and not found in any sources.