Hippo's

[Lee Priest]

Wha noise does a hippo make? Does it moo like a cow?

[New Forester]

Cattle do much more than moo! I can think of at least eight different sounds for different occasions, and only two or three of those could be described as a moo. Ducks don't all quack, either...

[wildwood]

Colinsin29 is right.... about the grunting anyway!
Pretty much like the sound pigs make while happily snorting away in mud, only a lot louder.