In many shuls that I have attended, the gabbai or someone else bangs once or twice on a table or on a siddur just before davening begins.
As I don't see this done everywhere, I assume this is a minhag. But, I have no idea when or why this began. Does it have any purpose? What does this bang accomplish that just the chazzan saying the first few words of davening can't already accomplish?
(I am not asking about the bang that some do as a reminder of a special "insert" like Ya'aleh Veyavo. That's meant as a reminder, and I gather that since they can't talk at that point, the bang somehow reminds people what to do.)