Max fan noise in sleep mode - PowerMac G5

[Mr. Ed]

In sleep mode, my wife's PowerMac G5 (2.0 x 2) sounds like a BIG airplane. And it won't stop unless I hold down the stop button. After restarting it (by pushing the same start/stop button), the computer works fine - fan speed is normal. Any ideas?

I used the extended AHT and got a passing grade on everything.