The (version of the) script that was released after the film, contains:
EXT. TATOOINE - ROCK CANYON - FLOOR - DAY
Artoo forces himself into the shadows of a small alcove in the rocks as the vicious Sand People walk past carrying the inert Luke Skywalker, who is dropped in a heap before the speeder. The Sand People ransack the speeder, throwing parts and supplies in all directions. Suddenly they stop. Then everything is quiet for a few moments. A great howling moan is heard echoing throughout the canyon which sends the Sand People fleeing in terror.
Artoo moves even tighter into the shadows as the slight swishing sound that frightened off the Sand People grows even closer, until a shabby old desert-rat-of-a-man appears and leans over Luke. His ancient leathery face, cracked and weathered by exotic climates is set off by dark, penetrating eyes and a scraggly white beard. Ben Kenobi squints his eyes as he scrutinizes the unconscious farm boy. Artoo makes a slight sound and Ben turns and looks right at him.
Hmm.... A "slight swishing sound" is not the same thing as a "great howling moan".
The Tuskens stopping quiet before the "howling moan" makes me inclined to say that they must have seen and been startled by Obi-Wan first. Obi-Wan makes a slight swishing sound as he moves around in his robe and the Tuskens got frightened by that and likely also from seeing Obi-Wan. This is important, because it says that the Tuskens don't just get frightened by the "great howling moan" itself.
So... the Tusken raiders hear the swishing sound from Obi-Wan approaching which frightens them to want to flee, but when they turn to see Obi-Wan, the sight of him frightens them enough to make them freeze for a moment and then the "howling moan" comes which triggers them to actually run away.
That still does not say who or what made the "great howling moan". It could have been Obi-Wan, it could have been one of the Tuskens in the group ("OMG, that is that crazy old Jedi! We are all going to die!"), it could have been a PA system that Obi-Wan had set up around the canyon, it could have been a Tusken sentinel on the rock above not frightened enough to not give a warning call, it could have been Obi-Wan's pet Krayt Dragon tied to a post behind the rock for all we know ...
The script does not contain the words "Krayt" or "Dragon" anywhere. That must have been made up afterwards.
