Changing the input voltage of oscillating fan
Reducing the input voltage of a motor cooled by a fan (as your fan's motor is) can cause it to not get enough cooling, slowly cooking the insulation until you end up with an electrical fire.  The fan was designed for a certain voltage, and it can be dangerous to run it on another voltage.
Also, unless you were to spend a lot of money for a Variable Frequency Drive, any adapter you would put between the fan and the wall would end up using the energy &quot;saved&quot; from running the fan at a lower voltage.  In other words, if the fan was using 100 watts before, in order to reduce the fan power consumption to 80 watts, the &quot;adapter&quot;, in the process of reducing the voltage, would use 20 watts.
I would recommend selling this fan and buying a new one that is not as noisy.