for me the beep occurs once every three hours right when the network switches to a new block.
I increased the difficulty from 256 to 512. It been 3 hours and no beep yet...
Several pages back when the B1 started arriving some investigation determined the random beeps occurs when there is a HW error.
The plot thickens.

One of my S3s reports 14,478 HW (I don't think I believe it) while another beside it reports 66 HW (might match the total number of beeps heard). They report 31,614 and 32,399 Accepted respectively, which I'm inclined to believe from the pool stats.
I haven't opened a case yet - gotta do unrelated things for a few hours.
Edit: detail I forgot to mention - both have "Beeper ringing" set to False and I'm not sure whether one or both actually make(s) the sounds.
Edit: more detail - both of mine have been running at 256 difficulty. They can go well over 3 hours without a beep though, then might have a few beeps in quick succession. I'll try 512 in any case.
Yeah once when changing settings it started up and did load of hw errors like that. A simple reboot sorted that out.