What would cause my PC speaker to sound a low frequency continuous tone as my system locks up? Generally what will happen is I'll notice the application I'm using will stop responding but explorer will be partially responsive, then after a few more actions explorer stops responding, and the last click will cause the pc speaker to emit a low constant tone as the machine locks up completely. The only resolve at that point is to restart.

Its not a thermal issue, I'm reading the cpu, and hdd temps with software and hardware monitors. AV and malware scans come up clean. I've swapped out my ram, reseated all my components. Used sfc with no results. chkdisk locks up at 3% and defragmenting does the same, but I can read the drive without trouble (I know this as I've done av/malware scans and I also backed everything up since this started happening) I'm mostly just interested to learn, if anyone knows, why the pc speaker would sound during the lock up.

windows xp sp2