Knock, Knock, Knocking on my Desktop!?!?

So it has been 5 months and 19 days since I returned to the U. S. and 6 months and 25 days since I had my last lecture in Germany, but I still find myself drawn to knocking on my desk top or the nearest hard surface at the end of a presentation or a guest lecture. Sometimes I catch myself as my curled hand gravitates towards the desktop. Other times I only notice when the rapping of my knuckles resound a much crisper applause than the multitude of polite clapping hands smattering their thanks/approval/appreciation/etc.

Just yesterday I caught myself knocking after a guest speaker's presentation, while the rest of the class clapped and it got me thinking. In university in Germany, it is common practice for the pupils to knock at the end of every lecture as a sign of respect. However in most of my lectures, many pupils carrying on full-blown conversations, send SMS messages to the universe, pass notes, and generally do not pay much attention to the professor and the professor just trudged on with the presentation. Here, however, if the disruption amongst the students reached even a fraction of that level, most of my professors would stop the lecture and reprimand the inattentive members before proceeding any further. And there is no knocking or clapping at the end of a typical lecture nor a bell, merely a collective, preemptive packing of book bags. So, with that, is knocking the equivalent of paying attention during lecture? Should they be cross-culturally synonymous?