"harmonics" noise

I had that same problem and at the same speeds. Went away at a higher or lower speed. Sounded like someone blowing across the mouth of an empty soda bottle. Chased the noise for a couple years and finally traced it to the drain for the galley sink! What happens is that if the trap for the sink is empty, the air blowing across the vent on the roof will cause the noise and give you the soda bottle noise. I now always tighten the stopper on the galley sink and if I forget and the noise starts up, I run some water down the drain and then insert and tighten the stopper.