Electrical hum noise from shower fan electrics box
Electrical hum like that can be a symptom of a loose connection in the wiring.  It's possible that something came loose when the wires were pushed up into the junction box.  If so, the worst case would be an electrical fire, so it's definitely something to have looked at.  Less terribly, you could lose power to the fan, where you'd probably have the electrician out again anyway.

If you're comfortable looking into it yourself, turn off power to that circuit at the service panel, open the cover and look at each of the connections.  Your profile doesn't say where you are, but in the US, wires are usually connected using electrical nuts.  Tug gently on each wire going into the nut -- there shouldn't be any give.  If there is, remove the nut, arrange the wires so that they're parallel, use a pliers to twist them together, then screw the nut back on.  People often wrap electrical tape around the open end of the nut, but it isn't required.