Open the side panel on the case (after unplugging the system from power, of course), and make sure that the rear exhaust fan is plugged in into a header called "FAN 1" on the mainboard. Refer to the other system if necessary.

Go into BIOS (hit Del at boot)-->PC Health Status-->Enable Q-Fan controller-->Optimal mode. Also do Enable Q-Fan for Case fan, set to Optimal mode as well. This will make both fans rotate slower, speeding up when necessary. Note: The CPU heatsink is also blowing hot air to the rear case fan, great thermal design.