What was the total load going through the main? I would expect to see the 100a cook off pretty quickly with 130a on one leg but if the total load is above 200 that could quite easily make the main go first.
The 100 could also be holding more than it should tho. You would have to look at the trip curve.
The hum itself is just a result of loading the coil in the breaker. You can also induce a hum in the cover or the enclosure.
