They *are* descended from avians (read: birds), so its a possibility-- but then again, turians do bear more than a passing resemblance to raptors (predatory birds of prey like hawks, falcons, eagles and the like), and many raptors *are* known for having high-pitched, screeching cries...Garrus may well be capable of -both-.But I personally like to think that the screeching is only going to be heard when he's doing something like shouting, yelling or screaming (or otherwise raising his voice); he probably isn't going to scream like that if he's just turning around to say something such as 'oh hey there, Shepard-- how can I help you?' (Which is most likely the gist of what he said, given that he's clearly shocked to see her screaming in reaction..."What'd I do, what'd I do, what's wrong?!" etc.)But of course, clicking and squawking wouldn't NEARLY be as funny here as the screeching...