Loud hums are usually either grounding problems or capacitor problems, particularly in the power supply. As you've already been told that there are power supply problems, that's the most likely cause. I'd guess that you're going to be changing all the electrolytic caps in that area for starters. It is nearly 40 years old and you may end up changing a lot of electrolytics throughout the entire organ too.
I don't know your electrical/electronic skills but be aware that there are plenty of lethal voltages where you will be working. I'm sure that our regular electrolytic cap expert will be along soon to give you some expert advice. If not, or if you can't wait, then look up some of the many posts on the subject from indianajo.