hstm,
It's interesting. If the CPU fan or the memory running there is a "fan not detected error message, for the system fan to be running at high speeds, it seems there must be a sensor/ fan control interlink triggering that increase in RPM.
You might go into BIOS and set the fan speed up three asterisks and see if teh behavior changes.
If the system fans are quiet, I'd say consider adding a fan card or change the GTX 750 to a fanned version.
If there is no change and the system fans are still roaring, then it's being triggered by the sensor fan interlink through BIOS - it may be panicing because it thinks there is no GPU fan- but I'm not certain how to solve that one. Anyway, as mentioned, these system were sold with fanless Quadro NVS..
What are the CPU tempertures running?
Cheers,
BambiBoom_Z