Is the head on the enlarger the original head that came with the unit when it was sold in North America?
If so, the problem may be a combination of two factors. First, as you have noted, the input voltage to the house is 184 rather than the nominal 220. If the step-down transformer was selected based on nominal ratings, that means that the actual input voltage to the enlarger head is 92 volts rather than 110. This means two things. First, the bulb will not emit as much light, and the color temperature will be warmer than specification. I suspect that is something that you can adjust for in using the enlarger.
Second, with reduced voltage, the torque produced by the fan motor is less than rated. If the rated input voltage is 110 v, the motor will only develop 69% of rated torque when the input voltage to the house is 184.
The second factor is that if the head is the original equipment furnished for use in North America, it was designed for 60Hz. The utility grid in Spain is 50Hz. This difference won't mean much for the lamp, but all things equal, the fan speed will be 5/6 of the design speed.
The combination of reduced voltage and frequency (relative to rating) will likely result in both abnormal vibration and heating in the fan motor.