My De Vere 4 x 5 fan noise is just the noise of wind. It worked as a commercial machine for about 18 years before I got it, I'm sure it's the original unit.
Basically, if the fan is making noise that isn't wind noise, then the fan unit is starting to go.
All of the De Vere units I worked with just made minimal noise and in a darkroom environment I only ever hear/heard the fan when I turn everything else off, with the enlarger being the only thing still on.
Mick.