Good, I’m glad you didn’t rip that gearcase apart. Like I say, those engines are normally pretty noisey. Your newer 30hp is probably quieter than the 1977 35hp.
Sometimes these engines have a bit of a "ringing noise" as well, never really figured out just what that is. But, I would definitely pull that flywheel and look for evidence of magnets rubbing on the components. Probably a good idea to remove the mag plate, clean and relube the bushings, replace the plastic bushing: 322435

There should be a plastic threaded/spring loaded screw that threads into the lower throttle gear under the vertical throttle shaft leading up to the mag plate. Screwing the spring loaded knob/screw in or out sets the idle speed. Unfortunately, these plastic screws are usually broken/jammed.