It's possible your computer is equipped with a temperature monitor on the CPU and it is heating up during the long render. The beep may be the temp alarm on the processor. Other people have had this problem and found if they opened something on top of poser (even just notepad) it would stop. I had this problem a while back and it turned out that the mouse driver was tweaking Poser's nose every few seconds and a new mouse driver solved the problem. Hopefully one of these will help.