My Mac Pro Two 3 GHz Quad-Core Intel Xeon 5300 processors running Snow Leopard with 12 GB 667 MHz DDR2 memory just started beeps or squeals when CPU usage is high. If I convert a couple of large Quicktime movies at the same time it will just squeal - and its not audio from the speakers because I shut those off. It also does it when I render 3D images that are large. I checked the Activity Monitor to see the CPU usage is high when this happens. Any ideas why?