If it weren't for it staying constant and not varying with rpm I'd say air rushing through the air cleaner or a vacuum leak around the carb, intake or even the vacuum wiper motor under your dash (if so equipped).
But it's loudest on the dist side, hummm, maybe a leaky vacuum advance canister?
I've seen electrical circuits vibrate at frequency's that could be heard, if you had a radio and heard it the capacitor/condenser in the dist could cause that. Might try replacing the condenser just for GP.
Good luck
Dave
