Power supply fan always on at 2800 rpm

OK, here's the situation. I moved to a new office and a new computer after someone left. It's a Mac Pro running 10.4.11 with Nvidia Quadro FX 4500. The Power supply fan is always running at 2800rpm from startup.
I've reset the SMC and PRAM
Plug directly into wall
tried Using SMCFanControl
Dusted it out (there was none)
No warranty left.

What do I do now? Please help, this thing is loud.

PS I don't know how the person who was here before me never thought this was annoyingly loud, she had a G4 before so that might explain it. And she was partially deaf, probably because of this machine.

UPDATE: I seem to have stumped everyone
Ran hardware test got error code 4M0T/1/40000003 : PS. So it's a hardware issue, but not sure what piece of hardware. Is the control for the fan, on the fan itself, or controlled on the circuit board?
Does anyone know where to find a description of Hardware error codes? I've googled to no avail.

According to Apple 4MOT is Fan Motors. Since you got a non-0 on the end, check to make sure the fan blades are unobstructed, that they're properly connected to the logic board.
Can't be sure without running ASD
solution: replace fans and retest. Still won't work, then replace the logic board.

I unplugged the fan and the Mac wouldn't turn on at all? weird. probably a safety issues so it won't overheat. It's definitely not obstructed, (see title). I have a feeling I'll be bringing it in to be fixed rather than purchasing items I may or may not need.

TwistyO said:

I unplugged the fan and the Mac wouldn't turn on at all? weird. probably a safety issues so it won't overheat. It's definitely not obstructed, (see title). I have a feeling I'll be bringing it in to be fixed rather than purchasing items I may or may not need.

Click to expand...

The error is for your power supply fans - you disconnected those? They don't have to be totally obstructed to give an error, but with what you've said it looks like you're replaced the logic board.

You never listed the year of your Mac Pro so I can't say how expensive it will be. Go eBay.