POLONIUS: The actors are come hither, my lord.
HAMLET: Buzz, buzz.
POLONIUS: Upon mine honour—
HAMLET: Then came each actor on his ass.
As far as I can remember (but there have been so many), every actor I've seen or heard says the words "buzz, buzz", the noise a child makes when they're copying the sound of bees, often with a pause between each buzz. But the Shakesyear blog proposes another option: he's making fart noises:
"Partridge flatly said so in Shakespeare’s Bawdy: the editors I’ve consulted venture that it’s “a sound expressing contempt,” which could well be periphrastic for “raspberry,” but they’re not saying in so many words. It would be extremely effective to play the line that way, as a perfect expression of Hamlet’s contempt for Polonius; that Hamlet’s next line is blatantly anal can only support this reading. But this cinches my point."I'm convinced too. Though my attempt at making the requisite sound leads to the noise a child makes when they're copying the sound of failure on Family Fortunes [via].