Weird smell from electrical disconnect box when appliance is on
Both of the things you report - the smell and the hum - are very likely indications of a bad connection or bad contact in the disconnect.  

Bad connections and bad contacts generate heat, the heat can damage the insulation of the wires, and even cause a fire.  

I'd leave that disconnect off and if possible turn of the breaker for the circuit until it can be checked.  You'll probably wind up having the disconnect replaced, hopefully the wiring can be re-terminated without replacing it.