Sorry for bringing up this old question, but I think this question deserves an in-depth analysis as I see similar question answered multiple times...
Noise, hum and ground loop are three related but different concepts.
First, noise refers to anything other than the signal in the sound system. Harmonic distortions ( http://en.wikipedia.org/wiki/Total_harmonic_distortion ) are noise, Other distortions includes intermodulation distortion, clipping (power supply), slew-rate related distortion (slow transistors), flutter & wow (due to medium), quantization noise (caused by encoding). For different kinds of audio distortion, see ( http://en.wikipedia.org/wiki/Distortion )
And then hum sound, yes, are usually caused by ground loops, and ground loop is usually treated by either treating the source or the recipient. The easiest way to solve amplifier-source ground loop problem is to connect them to the same ground (e.g. same power strip/same power outlet)
But for most notebook computer, the problem is often caused by the inadequacy of (1) transformer to filter noise (bad switching power supply comes to mind) - try to see the voltage from the notebook transformer with a CRO if available , (2) soundcard to handle noise in the power supply correctly.
Curiously, what sound card/pro interface are you using? Most pro external sound card had pretty good power supply rejection characteristics, i.e. filter caps in and out, external power supply for the sound card instead of using bus power, etc. And most important of all, remember to use the same power strip for both the sound card & the computer (and the amplifier/active monitor).