Knock, knock, knocking

This is an interesting monument for a graveyard. Who would be knocking on a gravestone? There's no door. How many wish entry?

Willie Shakespeare in MacBeth has knock, knock jokes. The hungover, or drunk, porter of the MacBeth's castle hears knocking.

Here's a knocking indeed! If a man were porter of Hell Gate, he should have old turning the key.
[Knocking from inside the castle]
[first of three jokes] Knock, knock, knock! Who's there, i' th' name of Belzebub? Here’s a farmer that hanged himself on the expectation of plenty. Come in time, have napkins enough about you, here you’ll sweat for ’t. ...

Bob Dylan wrote, Knockin' on Heaven's Door for the movie Pat Garrett & Billy the Kid. It begins:

Mama, take this badge off of me
I can't use it anymore.
It's gettin' dark, too dark to see
I feel I'm knockin' on heaven's door.

Knock, knock, knockin' on heaven's door (4x)

Knocking on heaven's door (or gate) is a common allusion. People were familiar with the phrase, Bobby put a song around it.

Both poets were borrowers. Hell-gate was a feature in mediæval mystery plays. It has to be assumed Shakespeare, and his audience accepted humour with death and fate. In Hamlet there is a gravedigger's scene. The gravediggers (listed as clowns) begin their discussion on whether the suicide, Ophelia, should receive Christian burial. They then riddle jokes:

1st Clown: What is he that builds stronger than either the mason, the shipwright, or the carpenter?
2nd Clown: The gallows-maker; for that frame outlives a thousand tenants.

After a bit, he first clown asks the question again. The second can't dig up an answer.

1st Clown: ... when you are ask’d this question next, say “a grave-maker”; the houses that he makes lasts till doomsday.