Noise from a my HP Pavilion Desktop?

I have two identical HP Pavilion desktops (D4999T)—one for home and one for work. Anyway, both of the PCs will occasionally make a screeching noise (through the speakers). I have different speakers, so I know they aren’t the problems. I have loaded all of the latest patches. I also have been to the HP site and google’d the problem, but I can’t find anything. Does anybody have one of these?