This is what's in the manual:

Q: What do the beeps emitted during the POST mean?
A: The following Award BIOS beep code descriptions may help you identify possible computer problems. (For reference only.)
1 short: System boots successfully
2 short: CMOS setting error
1 long, 1 short: Memory or motherboard error
1 long, 2 short: Monitor or graphics card error
1 long, 3 short: Keyboard error
1 long, 9 short: BIOS ROM error
Continuous long beeps: Graphics card not inserted properly
Continuous short beeps: Power error

I don't think any of these describe my situation, where it's one continuous long beep that doesn't end until the power is shut off. It's not a series of long/short beeps, just one beep that goes on and on.