Well I have the novel. It says first of all that Artoo heard it too, and actually specifies that this means it wasn't "an auditory delusion" but a real physical sound. In the novel Kenobi does it again, after he says the sand people might be back--to which Luke recognises it as a Krayt dragon call.

From pg 72 of the 1994 (?) reprint:

"...The sand people are easily startled but they'll soon be back and in greater numbers. A landspeeders not a prize readily conceeded, and after all, Jawas they're not."
Placing both hands over his mouth in a peculiar fashion, Kenobi inhaled deeply and let out an unearthly howl that made Luke jump. "That ought to keep any laggards running for a while yet," the old man concluded with satisfaction.
"That's a Krayt dragon call!" Luke gasped in astonishment. "How'd you do that?"
"I'll show you sometime, son. It's not too hard. Just takes the right attitude, a set of well-used vocal chords, and a lot of wind."