What is an example of onomatopoeia used in the story ''The Secret Life Of Walter Mitty''?

James Thurber uses many made-up words in "The Secret Life of Walter Mitty," but only two or three seem to qualify as examples of onomatopoeia, which is the creation of words that imitate natural sounds. The word "pocketa," which is always repeated at least three times with hyphens between each, is used in several of the fantasy episodes of the story. The first time is when Mitty is driving his wife to town on a...

James Thurber uses many made-up words in "The Secret Life of Walter Mitty," but only two or three seem to qualify as examples of onomatopoeia, which is the creation of words that imitate natural sounds. The word "pocketa," which is always repeated at least three times with hyphens between each, is used in several of the fantasy episodes of the story. The first time is when Mitty is driving his wife to town on a shopping trip.

The pounding of the cylinders increased: ta-pocketa-pocketa-pocketa-pocketa-pocketa.

The pocketa-pocketa-pocketa sound increases because Mitty is unconsciously pressing harder on the accelerator. He is imagining that he is piloting a "huge, hurtling eight-engined Navy hydroplane" through an approaching storm, until his wife intrudes with the shrill voice of reality.

"Not so fast! You're driving too fast!" said Mrs. Mitty. "What are you driving so fast for?"

Then when Mitty is the famous surgeon in the operating-room episode:

A huge, complicated machine, connected to the operating table, with many tubes and wires, began at this moment to go pocketa-pocketa-pocketa.

The situation becomes more critical when the machine starts going:

pocketa-pocketa-queep-pocketa-queep

"queep" would be another onomatopoetic word.

When Mitty is fantasizing about being a World War I flying ace:

The pounding of the cannon increased; there was the rat-tat-tatting of machine guns, and from somewhere came the menacing pocketa-pocketa-pocketa of the new flame-throwers. 

There's that "pocketa-pocketa-pocketa" again! And the "rat-tat-tatting" is a third example of onomatopoeia.

Thurber loved to make up words which are not technically onomatopoeia. In the operating room scene Mitty is told that the patient has "Obstreosis of the ductal tract. Tertiary." And moments later he is told that "Coreopsis has set in." This is all nonsense, but the kind of nonsense that Thurber admirers love.