You are certainly curious about pelicans aren't you? Two postings on the same subject...
Actually adult pelicans seldom make any sound, but when they do it is a low grunt. Not a quack. Baby pelicans, on the other hand, squeal like a stuck pig.
I know this because I am am the pelican keeper at Birdworld in Farnborough. I can also tell you that when the adults feed their young they regurgitate a bit of food into their beaks and the children all stick their heads in and gobble it up. The greedy little tykes. Basically eating sick out of someone's mouth - but that's nature for you.
Wishing you all the best in going on to learn more about this wonderful (and underrated) marvel of the bird kingdom.