We first hear this sound in reference to the "Navy hydroplane" that Walter steers through a violent storm; he imagines it's the "pounding of the cylinders" (1). Later, in Walter's fantasy surgery, the "new anesthetizer" gives way and makes the same sound (6). Finally, when Mitty imagines himself a British pilot, flame-throwers make the same noise. It might be that this is the sound of the car engine, which Walter first hears when he's driving into town with his wife – although we do hear the "pocketa pocketa" again when Walter is sitting in the lobby, not in his car.
Regardless of where the "pocketa pocketa" originates, we do know that the sound provides a solid link between the real world and Walter's fantasies, as well as a common link between his multiple fantasies. It's one of the many comic elements of the story – the same funny sound cropping up over and over again – and it lends a tangible, permanent element to Mitty's various fantasies.
Mitty's Bumbling Jargon
Mitty fantasizes about being a Commander or a pilot or a surgeon or a crack shot, but in fact, he is none of these things. And as such, he doesn't really know what he's talking about. When he imagines being a surgeon, for example, he fantasizes that "Coreopsis has set in" (6). This isn't so much a disease as it is…a plant, rather similar to a daisy. And the gun Walter imagines in the courtroom? A "Webley-Vickers 50.80," or a gun with a three-foot barrel (10). Again, this is part of the humor of "The Secret Life of Walter Mitty." His fantasies read more like exaggerated parodies of adventure stories than like genuine drama.
Overshoes and Gloves
Mrs. Mitty badgers Walter to buy overshoes at the store; she also insists that he wear his gloves while driving. It seems to us that a lot of what she does to Walter has to do with sheltering him from the world. After all, she won't let him do simple things for himself (like take his own temperature, or take the chains off his car, or make basic decisions), and this is a big part of the reason he spends so much time fantasizing. If this is true, then the gloves and overshoes might be symbols of the way Mrs. Mitty tries to shield or protect her husband from the world.