I have a Mac Pro 3,1, GTX 570, 32 gb of RAM, running 10.11.6. No matter what I do the fans stay on. When it goes to sleep, the monitors go dark, but the fans stay on, and I dont get that nice pulsatile sleeping power light. I have done the terminal command: pmset -g assertions and there is nothing preventing the system from sleeping. I powered down and reset the SMC/PRAM without improvements. Any thoughts?