Have someone check the rectifier diodes or bridge, whatever is used, and or power supply caps.
Transformer hum is mainly mechanical, usually involving loose core laminations in typical transformers, but can also be loose mounting or "sounding board" affect on chassis, covers or other susceptible components. It occurs because the reversing 60 Hz magnetic field tends to move (vibrate) the ferrous transformer parts and this is coupled mechanically to the mounting. This can be a quality or age issue with the transformer itself. Generally, damping will probably be the primary suggestion you get. Mechanically isolating the transformer, adding material to vibration prone areas, or just isolating the unit from it's supporting structure (shelf). There will probably be many specific suggestions forthcoming.
I had this problem myslef. The power supply would hum so bad that I could here it over the music. It took a lot of questions and looking but I found the problem to be that there was something else that was sending contaminating power back through the electrical system. At the time I lived above a bueaty salon. When I moved the problem went away. If this is the reason a power condiotiner may help. I would try plugging it into a friends system and seeing if the problem is sill there..
Try transfering the vibration away from the audio path. If you give the resonance frequency a place to go other than the audio path it will. The most effective unit out there deigned to do this is a Sistrum stand anybody that has heard one will agree. Look it up on the audiopoints website @www.audiopoints.com. However you should still fix the source of the problem. Most times lifting the ground is a simple solution and cheaper than a power conditioner. Still try the rack even if to fix the hum.
Flying, Sndsel has hit it on the head! It is simply loose core laminations in your power transformer. This is a very common problem so don't feel alone. Placing sorbathane pads under the transformer will most likely do the trick. Your problem has nothing to do with a bad power supply or bad AC power. Please do not run out and purchase an expensive line conditioner as it will not fix your problem.
Where do you purchase sorbahane and what form/shapes does it come in? I have seen it mentioned in other threads, but only recall seeing some little half sphere footers of sort that I think were made out of this substance. I would like to try it out on the power supply of a tube amp that I have and would like it in washer form if possible for a rack that I am planning.