Does the loud sound seem like a hum as soon as you turn the organ on?
If so, it will require a service.
Let me know what ZIP area you are in and I can recommend a nearby servicer.
Ok, well you might just have a bad connection somewhere. Turn the power off, and remove and replace every connector, and flip all the tabs back and forth a few times. You can use a pencil eraser and rubbing alcohol on them to clean the electronic contacts.
Over the years the brass and tin contacts and switches corrode, and the electrolytic capacitors dry up. Dried up capacitors can cause components like the rectifiers, resistors, regulators, and even transformers, to burn out. These faulty components can in turn cause the humming sound. The large electrolytic capacitors near the transformer are most likely to dry up or blow first.
You should also check that the power supplies are somewhere near the standard voltages of +-25V for the power amp, +-5V or +-12V for the logic sections. Let me know how you get on.
Please reply back if you need clarification on any of the above, or even if you just want to say "thank you" :-)