Computer fans use to run at 12Volt, but most, as for today, use to allow 9Volt or even less to slow down the fan speed (RPM, Revolutions Per Minute).
In cases of too low voltage, the fan stops, but I can see it "trying to start again". For example: my Tacens 9dB fan stops at about 10 Volts, but to start it again, 10.5Volt is not enough, and the engine tries to move the fan (I can see a small movement as an "attempt" to move) each 1-5 seconds, but it does not succees, so the fan keeps at 0 RPM.
Maybe that "attempt" to move could damage the internal engine of the fan when it last for hours or days?
UPDATE: Why would I want to stop (some of) my fans:
- Less noise when temperatures are low and fans are not needed.
- Dust reduction (fans acumulate it).
- Less consumption.