I always thought that it was the little can device that made that noise, its never even been suggested to me that a human made it. But now that it has been planted in my head... I sort of buy it.
I'm guessing this was unintentional and Spielberg just left it in because it established what a wanker this character is. Probably the most spot on character from the book in this whole movie.
I thought the whole scene here did a perfect job, simply from the way props were used. His entrance with that huge unwieldy bag and the whole eating thing made for some fun discomfort about both of these gys. Oh and let's not forget "we've got Dodgson here!".
I re-watched it in high-definition for the first time in years, the plot is both dumb and quite possibly one of the most plot hole-filled big-budget blockbusters ever made. But the special effects are still pretty great (aside from a few pure CG bits that are poorly composited).