hstm,
It may be that BIOS has a setting that detects the presence of a GPU fan and signals the fan controller to increase RPM, communicating through the various fan connections. If however, there is no return signal for a fan, it increases the speed of other system fans. This is guessing.
However, HP z-series are sold with fanless versions of Quadro NVS, but these are very understressed 2D-only GPU's, so it may be that with a 3D gaming GPU, the system temperatures are rising to a level that triggers a protective mode. Have you checked the temperatures in the z420 for CPU and GPU? It may be that a GTX 750 under load is simply running too hot. Looking at the HWMonitor in my local z420, the Xeon E5-1660 v2 is 45C and the Quadro K4200 is 48C. The fans are all running at about 2200RPM- 31% of maximum. > What do you see on your z420?
It may be easiest- and safer- to change to a GTX 750 having a fan. If, however, the 750 is running too hot and changing it is impractical, you can add a PCIe fan card set to conduct air to the chipset/ cast heatsink side if there is an open slot.
The z-series are very quiet. We have two z420's and a z620, all with Quadros: K4200, K2200, and 4000 and these under the various desks are inaudible with any sound present of any kind With only the air condtioning on I can't hear these systems from 1' away. It's never quiet enough in the office to hear these. It's a different story with the Dell Precisions (390, T5400, T3500, T5500) but they are not too bad either. The T5400 is the loudest as the 32GB of DDR2 RAM runs very hot and the system has enthusuastic memory fans.
I think that by type, GPU fans are the quietest fans anyway- it's the CPU fan that makes more noise. A fanned version would certainly be quieter than the roaring fsystems fans.
Cheers,
BambiBoom_Z
