Humming sound is not a good sign , but not a bad sign either.... You could say its normal
its very common for Corsair PSU's for example,they tend to make a very light humming sound when under alot of stress, from what ive heard, no PSU's have died from that sound, its the squeeling sound you want to watch out for, when that happends, you need to replace it asap.
my corsair vx550w does a light humming sound sometimes.
But as i said, thats nothing to be worry about right now, but when it starts to squeel or make other noises then just humm's, then its time.
I discovered my humming sound from my PSU after i installed my gtx470 and i removed some fans from the case and it was dead quiet except that humming, then i knew.