What are the main sources of humour in this story?
The Miller and the Carpenter have words on the trip, so when the Miller's turn comes to tell his tale, his main character is a carpenter. The Miller makes the carpenter rather dumb and gullible in the story to get at the Carpenter on the pilgrimmage. In the story, the main humor is that the carpenter is being made a fool of by his own wife and her lover. In order to get some alone time, the wife and her lover tell the carpenter that there will be a flood and that they must sleep in huge tubs secured to the roof so that when the flood comes, they can simply cut themselves loose and float safely away. The carpenter climbs into his tub and quickly goes to sleep. His wife and her lover run back to the bedroom for some alone time. However, it is complicated by the fact that Allison (the wife) is also pursued by another man. He serenade her at the window and begs for a kiss. She tells him to close his eyes, and Allison's lover allows the serenading fool to kiss his backside through Allison's window.
The whole story is full of deception, jealousy, lust, and lack of marital loyalty. It is not a nice story, but amusing due to the idiocy involved.
The sources of humor in "The Miller's Tale" are bawdy, farcical, borderline scatological, and slapstick.
We remember that the Miller has been drinking and that this colors his tale. His story displays bawdy humor because it deals with sex and nudity in a comic way. Alison and Nicholas trick Alison's elderly husband, John the carpenter, so that they can sleep together. It's farcical because it deals with over-the-top situations: how many men, like the carpenter, are really going to be convinced to spend the night dangling in a tub because they believe someone's prediction that the earth is about to be flooded again, as in Noah's day? While scatological humor deals with feces, we come very close with farts and anuses, as Absolon, Alison's would-be lover, is tricked into kissing her anus, and Absolon, for revenge, sticks a hot poker into Nicholas's anus after Nicholas farts in his face. The humor is slapstick because it involves physical encounters and physical embarrassment: not only do buttocks play a strong role in the humor, John is laughed at by everyone and considered insane for thinking a flood is coming.