I have an HP Z840 Workstation. I could not understand why the fans always ran fast. I assumed it was due to running Red Hat Enterprise Linux 7 on the workstation, but other Linux distros had done the same.
I recently plugged in my audio speakers into the built-in 3/8 inch jack in the back of the machine. When I did this, the fans immediately slowed down to the normal, and quiet, speed. I've monitored the CPUs' temps and all temps check out, even under heavy load. Under load, the fans speed up a little bit, but not nearly as much as when the speakers are not plugged in to the workstation. So the fans all appear to be working well.
I have a small video of this because I don't think anyone would believe me. I'm not sure if I can attach it here.