We know 'hear someone/something doing something' is correct. But, I suppose, the question is, is 'hear someone/something do something' also correct? And if yes, is there a difference in meaning?
(a) I heard him crying for help. Then there was silence.
(b) I heard him cry for help. Then there was silence.
What do you think? Is (b) OK? Personally, I think that if it is a repeated cry for help, (a) is correct, but if it's a single cry for help, (a) or (b) is correct.
In the example given, a hum is a continuous sound. This is comparable to(a) above - OK, the cry's repeated, the hum's continuous but they're comparable to the extent that they are continuing - so for that reason only, I would say 'coming'.