The "Feeding time at the zoo" sound.
It happens twice in the Classic 39. Once in "Bensonhurst Bomber" when Norton punches Ralph in the stomach; then again in "Oh, My Aching Back", when Uncle Leo visits and slaps Ralph three times on his injured back.
To me, the noise Gleason makes is absolutely the funniest thing I've ever heard. The stomach punch is especially hilarious because he lets out about four of those sounds.