Hey, -Sounds like it might be the old favourite 120Hz rectifier hum, in which case you probably do need to replace a couple of caps.
Not the tonewheel caps in the link posted earlier on, but the rail electrolytics. These will probably be large in physical size, and will almost certainly have the highest voltage rating, as well as the most lethal voltage unfortunately...
However...
I doubt they're leaky, -they might be of course, but it's much more likely that they're just low value. A capacitance meter will tell you fairly quickly, but not all capactiance ranges on multimeters go all that high. -Assuming that they're low value, the reason it's huming is because they've lost sufficient capacity to be able to sustain a significant charge, and they aren't therefore able to smooth out the ripple. -the side-benefit is that they probably won't be able to sutain much of a charge to kill you. All the same, leave it off and unplugged for a good while before you work on it!
(you can never be TOO safe!)
Bear in mind that all aluminum electolytics exhibit a tiny amount of leakage, and -as I've already suggested- there are other ways that a cap can go bad, the most common with this type of electrolytics is for the electrolyte to dry out, then they just go low value. Leakage in thses sorts of failures can often go down, so a simple leakage test won't give you even the slightest hint, and I'd bet at least an hour's pay that you've got low-value reservoir caps if you have a 120Hz hum.
You can try it yourself, but for goodness' sake, BE CAREFUL!!!
Note that I earlier wrote "unplugged" and not "switched off". If your soldering iron tip is grounded (They basically alla re nowadays) and you try and desolder a rail cap that is holding a charge above ground potential -which it is their job to do- then just toouching it with your iron will cause the immediate discharge that Sahib warned against. unplug the unit from the power AND ANY AUDIO CONNECTIONS to or from it, otherwise it could be ground referenced through these connections, which would be a problem.
Keith