55% manual drops speed down to less then killer sound maybe 3300 - 3500 rpm
35% manual gets you to 2600 - 2800 but you must lower freq.
Depends on particular unit (possibly batch). Depends on room temperature. Depends on air flow in room. I run all my fans at 35% and this is OK in my room provided that the temperature in the room stays below 30 degrees C, even on units that are slightly overclocked.