Humming may or may not be significant. Really hard to say, for sure. Most humming sounds from breakers are just annoying, and not particularly significant, but some are symptomatic of a bigger problem. There’s no way to tell the two apart by the hum alone, without further investigation. I would add that in my personal opinion, Murray is a trash brand, so I’d suspect poor design rather than a real problem. Just hard to say without more effort put into it, however. You might also consider the possibility that the breaker was just bored by being tripped off, so it decided to hum.