ῥοίζησεν
The A scholia explain that this verb means to “make a nonverbal sound, which we call συρίζειν,” a verb that means to whistle or hiss. Compare the related noun at Odyssey 9.315: πολλῇ δὲ ῥοίζῳ πρὸς ὄρος τρέπε πίονα μῆλα (“With much whistling (?), he [the Cyclops] was turning his fat flocks to the mountain”). At Hesiod Theogony 835, ῥοιζέω is used of the monstrous Typhoeus, and because this monster has snakes as part of his body, ῥοιζέω is generally assumed to mean ‘hiss’. Both this verb and that to which the scholia compare it seem to include both meanings, and we can understand both as sounds the mouth makes by moving air through the teeth and/or lips. So whichever sound we imagine Odysseus making here, the key for the poetics of ambush is that it is a single, quiet, nonverbal sound, lest the other sleeping Thracians awaken before he and Diomedes make their getaway.