replying to heathcliff, Leo wrote:
The best is to buy second box fan and connect wires in series, so these two fans
become very quiet and you have a lot of speed options: from knob position 3 on
both to 1 on both. Even 3 on both is quieter than single fan on 1.
I also tried light dimmer but then fan motor was producing loud buzz, I guess
because dimmer output is not sine wave, so that was a bad idea.
Now, I am going to buy AC Motor Speed Control
and hopefully it will work better than regular light dimmer.