gateway e4100 shutdown problem

I have a gateway e4100 machine with windows xp loaded onto it. After powering on, it shuts down after about 10 minutes and begins chirping or ticking like a clock and the green light flashes in tempo with the chirp.

I thought this was a thermal problem so I cleaned the fan and heatsink assembly to no avail. I have tried unplugging the computer, holding the power button to drain the PC, then powering back on, but still no help.

Curiously, when I opened Windows XP in Safe mode, I didn't experience this shutdown. When I restarted in Normal mode, the problem returned. Any ideas? Thanks much.