Welcome to the Forum!
You'll find some very nice people here, united under a common cause--preservation of a great instrument! Everyone was very kind to me when I was restoring an old M-3 a few months ago. I'm no expert, but here are a couple of preliminary things that would be good to know:
--Is the "C note" the same volume as the pedal note you're actually playing?
--Do you notice any kind of stray tone when playing keys on either of the manuals?
--Are you using the organ near any fluorescent lights, lights with dimmer switches, ceiling fans, et al?
I ask these things because I was having some problems with 60 Hz hum, which happens when the miles and miles of wiring inside the Hammond "picks up" some electrical buzz from the outside world. I've always thought it was mechanically *impossible* for the Hammond to be anything but monophonic--one note only--from the pedals.
In my case, there was a "low hum" happening whenever I played *anything*--pedals or manuals. It was markedly lower in volume than my "real" notes, but it could easily have been mistaken for an honest-to-goodness "unplayed note," if that makes sense, which is why I asked what I asked...
Don't give up--there are plenty of helpful souls here...