The power output from normal generators is actually pretty noisy, it is not a regular sine wave like you get from the power company, and may have high frequency harmonics. According to one source I found, total harmonic distortion (THD) from a generator can be >15%, where normally acceptable THD is 5-6%.
Here's an animation from wikipedia, showing a normal sine wave, and then the effect harmonics have on the overall signal:
In an AC motor (simplified explanation) the sine wave directly runs the magnets in the motor, causing it to spin. The harmonics effectively reverse the polarity of the magnets mid-rotation - it's as if you're flicking back and forth between forward/reverse hundreds of times a second. I believe this is what causes the hum.
As for why your tower fan works, hard to say. The motor design may be less susceptible to the effects of harmonics, it may have electronics built into it, or they may be DC motors.
So the solutions:
- Don't run your fans while on generator power
- Use a power conditioner after the generator (which may or may not help, depending on the quality of the incoming power)
- Get an "inverter generator". They are more expensive, but produce a pure sine wave output, and are more efficient because they are capable of lowering their speed to match the load