Help identifying a ground loop hum

Ok, I'm talking basic--I now wouldn't know one kind of humm from another if it bit me in the face. I'll describe what I've got, someone please tell me what it is... About 50% of the time that the pre-amp is on, there is an audible, electronic hum that comes directly from its power supply. The power supply hums--not all of the time, sometimes at first, sometimes only later, sometimes not at all--but it defintely hums. Is this a ground loop hum? Is it just sporadically noisey, and them's the breaks? Is there a problem with the pre-amp? Many thanks.
If you have cable TV, disconnect it and see what happens. Also, if your preamp is tubed, change it (them) and see if hum is gone. Good Luck!
Mezmo: A little clarification, please. Are you saying that the "hum" is physically generated by the external power supply and not heard through the speakers? If not through the speakers, then the transformer in the external power supply is the likely suspect. You can try spikes under the unit and weight on top, as long as you don't cover vent holes. If necessary, the transformer can be suspended from the chassis by rubber grommets to minimize vibration coupling. If the hum is simply due to a cheap transformer or an undersized transformer, a good tech should be able to recommend an appropriate replacement (probably toroid) to eliminate the hum. Sorry for the long answer. Hope it helps.
Mezmo : from your note it seems that the noise comes from the power supply. If caused by a ground loop or if it´s the transformer itself, just disconnect everything from the preamp and if it still "hums" as you described then you know the problem is isolated in this unit meaning is not a ground loop because there´s no loop. So the transformer as JCBtubes suggested is the source. On the other hand you don´t mention if you have power conditioning. You might consider this as a possible cause too (lack of it). Your comments
Thanks for the replies. More details I can do. The preamp is a VTL 2.5. No cable TV. Yes, the tubes are noisey--I've replaced a particularly bad one and am considering switching out the stock tubes all together--but that's another story. The hum I am concerened about here is from the power supply and happens when the preamp is on, regardless of whether any of the other components are even turned on (so, no, nothing to do with speakers or sound reproduction in any way). The noisey bit is the torroidal jobbie towards the back of the preamp (power supply?)--so no, it's onboard, not seperate. I will un- and re-plug everything in various configurations to see if that makes any difference (thus checking for a loop by removing eveything else...). Currently no power condiditoning, so that may be an option as well. In fact, I must admit that my current power arrangement stinks. I'll also experiment with various dampening and isolation tricks. All good suggestions, and I suspect I'm on the right track. The one piece of info I'd still like to get my hands on, though, is how precisely a ground loop manifests itself. For all the talk about them, I have yet to see a simple explanation of what exactly makes the noise (I know why (sort of), and how to fix it, but not what). If a gound loop causes distortion in the reproduction of music, or any kind of acoustic artifact that is then amplified and broadcast by the speakers, then I don't have one and I am wasting my time trying to fix one. If, however, the "hum" from a ground loop manifests itself by causing the actual physical compenents of the electronics (such as the internal power supply on a preamp) to, I assume, vibrate enough to create an audible (and annoying) hum--then treatment for a ground loop is the thing for me. I guess, if you'll pardon the excess verbiage, my question is a simply as, "which is it?" Thanks again.
I am not sure I can help, but it seems clear to me you do not have a ground loop hum at all since you would hear this through the speakers. The hum you do have appears to be an unhappy transformer. This might be caused by a number of things. Deterioration of the transformer, a fault in the unit causing the transformer to draw more current than it was designed to, low voltage coming out of the wall socket, interference from another electrical field (such as if you have placed another component too close - like underneath or on top), or interference from another appliance in your house or in the neighbourhood - any of the above, and probably some others too. This may give you some things to check. Also try plugging the unit into a wall socket in another room, or a friend's house. Failing that, take it to a technician.
Much thanks. On the road to salvation.