I think the humming is the contractor rapidly switching between on and off, so it appears to be humming.
Crossy will know more, but if you have under and over protection on your mains incoming, it can be a pain when the trip points you set are correct, but the incoming voltage falls outside these limits.
You could have the 3 phase mains come into 3 stabilisers (one for each phase). Then when an incoming phase voltage drops, the stabilisers corrects this so you still have 220V output.
You could then just install over/under voltage protection on say your swimming pool motor feed. 🙂