What made the moaning sound that scared away the Sand People? Most people think that it's either a bantha that got scared of Ben-Kenobi coming, or Kenobi used the Force to make a sound.
Obi-Wan Kenobi mimicked a Lesser krayt dragon hunting cry when he scared the Tusken Raiders away from Luke Skywalker.
Luke Skywalker: "What made the Sand People leave?"
Obi-Wan Kenobi: "I imitated the hunting cry of a krayt dragon. Their imaginations did the rest and they took to their heels."
There are even two different versions of this cry; the theatrical version and the Bluray release version here for comparison
Although a live Krayt Dragon is never seen in A New Hope (or any other Star Wars movie), C3PO comes across a partial skeleton of a Greater Krayt Dragon while he's out wandering in the desert.
This is covered in the official
Alan Dean Foster George Lucas novelisation of Star Wars Episode IV : A New Hope.
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 gaped in astonishment. “How did 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 cords, and a lot of wind. Now, if you were an Imperial bureaucrat, I could teach you right off, but you’re not.” He scanned the cliff-spine again. “And I don’t think this is the time or place for it.”