There is one family that I occasionally eat with that has a minhag of banging on the table during Birkat ha-Mazon. I was wondering what reasons lay behind this custom.
I do not know if this is relevant, but there is a particular rhythm that they have (the following description only would apply on Shabbat):
Bang #1 Middle of Psalm 126 right after "הגדיל יי לעשות עם אלה" (Hashem has done great things for them)
Bang #2 Near the end of Psalm 126 before "בא יבא ברנה"
Remaining bangs: Every off-beat during final paragraph of abridged version (Oseh shalom bi-m'romav etc.)
I do not think it is just a custom they made up themselves, since one time at the local Orthodox shul I saw an (unrelated) guy do the two bangs on the introductory psalm, but not at the end (presumably since we said the full version?). I noticed it because usually at the shul nobody does any banging at all. Any information you have for an origin or reason would be interesting.
This post Drumming on tables on Shabbat is kind of relevant as to whether this is permissible, but that is not really my question.