For an absolutely silent mining operation (fan running at 1k RPM or so, basically inaudible compared to the quiet 2k RPM or so of ECO) set brd_ocp and osc to 30. You'll get about 1.3TH/s with no noise. If you want to go higher and keep the noise down, you can add more external fans running at very low RPMs. I'm in the process of designing this solution.
I agree with external fans. My Apollo sits on 4 feet that I cut from some rubber and one fan has been slid underneath and one laid on top. I hardwired both these DC fans to a USB cable that is plugged in into one of the Full Package's USB ports, so they run at roughly half speed (5V of their 12V max rating) and it hashes 3TH/s pretty silently at 200W. It is not inaudible, but it was a very easy quick setup with no software modifications. It's all stock on 'Balanced' mode and the fan adapts itself downwards after adding quiet external fans.