Optiplex 760 powersupply fan running loud

I have an Optiplex 760 that I have loaded with Slackware 13. The power supply fan runs so fast its hard to use the machine.

By contrast, i have a GX620 running the same OS and i can hardly hear it at all.

I've checked the BIOS to ensure fan override is off and I've verified that the CPU temps are low (20-25C). I also loaded kubunty 9.04 livecd and got similar results.