1 {scene: A front door in Ancient Rome.}
1 [sound]: Pulsus! Pulsus!
2 Charity Collector Guy: {As Marcus opens the door} Ave! I'm collecting for the fund to prevent Latin from becoming a dead language.
3 Marcus: {to Julius} What did he say?
4 Julius: I don't know; it's all Greek to me.

My meagre Latin skillz (consisting of a copy of Collins Latin Dictionary & Grammar and Google) came up with either "pulsare! pulsare!" or "pulsus! pulsus!" for "knock! knock!"

I went with the latter simply because it sounded better. I also formulated a vague theory that "pulsare" is a verb form and "pulsus" a noun form, and that when saying "knock knock" the word "knock" is being used as a noun, not a verb. But not even I'm convinced by that.

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2016-07-19 Rerun commentary: In hindsight, maybe this joke would have worked better if the Charity Collector Guy had actually spoken in Greek.

As opposed to "English in a Roman style font, with the 'u's replaced by 'v's." Which was kind of meant to indicate he was speaking Latin - whereas Marcus and Julius are clearly speaking English.

Yeah, yeah, I know it makes no sense.