Hissing and Humming noise

Hello

I recorded my laptop fan sound as a noise and i also generated a white noise signal in matlab using randn.m. My question is between this two which one is a humming noise and which one is hissing? I am not sure whether my question does make any sense or not.If i want to compare both the noises what should be the factors? It would be also appreciated if the answer also explains differences between humming and hissing noise. I did plot both the waveform for a common interval from 0.5 to 0.6 second. I found the white noise signal is more crowded than the noise signal within the same interval. Why it is so?

Thanks a lot.

Nuzat.