Hey, good timing with the ears.
From your statement here, it sounds like the buzzing is coming from something like an alternative relay.
The Relay is energized and hums when on 1,000 points, and off for 100 points. These Relays are
made to remain energized without doing any harm. With the playfield up, you should be able to
hear and see which relay is energized and making the buss. I think I have placed a small piece of paper
plate (the very cheap kind) between the plate and coil to reduce the buzzing..
What's the name of the game again, Top Machine?

I just checked your schematic link. From your description, I believe it's the "Change Relay"
that is doing the buzzing..

It could be on the bottom board, but I'd say most likely on the underside
of the playfield..

Yea, you don't want that coil staying energized for long.
Seems the 1,000 point relay is staying energized which is also
in the head. Hard to believe it hadn't burnt out after playing
a full game. I'd find the 1,000 point relay, then next time you hear
buzzing, check if that relay is remaining energized, but I wouldn't
leave it energized for long..

You mean at start up?
Yea, my guess would be there's a stuck (closed) switch on the playfield that scores 1,000 points.
I'd check all the switches that score 1,000 points. There's also a spinner right? I wouldn't think
that it's not it, but might as well check and make sure the spinner switch is normally open..

When you start a game, the ball doesn't kick out, but does the score motor run and keep running?

It sounds like the Coin Relay is energizing the Score Motor, but the Outhole Relay isn't energizing at all.
You'll want to make sure the Bonus Unit is resetting all the way home and it's wiper has good contact
on its wiper board, and also the Outhole switches are making good contact when the ball is in its kick out
position and also a Make/Break switch is clean and adjusted properly on A2 Score Motor. A Make/Break
switch are three switches in the set where the center switch is either touching one of the outside switches or
the other. The Score Motor label may still be attached on the inside of the cabinet near the Score Motor stating
where the set of switches on the Score Motor cams are, and what the do.
Note: It's always a good idea to make sure the screws for the stacks of switches are tightened before
checking and adjusting..