the buffalo roaring sound effect.
I couldn't place it at first, but now I know:
Is it not the exact same sound effect used for the Loch Ness monster in the DOCTOR WHO episode "Terror of the Zygons" 2 years earlier?
A few other scenes had it roaring like the monsters on the STAR TREK episode "Galileo Seven", and once or twice they reused the roar King Kong made in the '76 movie.