The Q switch and the lightbox ball shooter track switch are the only switches that send power to the 100 point ball solenoid. You can use a piece of tape, business card or unused shrink tubing to block these switches. If there is still power going to the 100 point ball solenoid after you "block" these switches, then there is a short somewhere in the circuit. Also, check the switch lugs at the switch stacks...

Q relay switch - orange/blue wire and red/white wire
Lightbox ball shooter track switch - orange/blue wire and red/white wire

Thoroughly clean and adjust all the pop bumper switches. Also, look at the switches on the Q relay. Are they clean and adjusted properly?

Since those are the only two switches (Q and track) that can send power to the 100 point bell solenoid, there must be a short in the circuit.

On this machine the pop bumpers score 10 points when lit and 1 point when unlit. Does the buzzing start when the pop bumpers are lit or unlit when hit?

If the Q relay coil is not energized and the track switch is open when the buzzing occurs, then another source is sending power to the 100 point ball solenoid. Maybe a Jones plug has been mis-inserted. Check all Jones plugs on the machine to see if they are properly connected...

When the buzzing begins, are relays M or N energized? Also, is there anything happening to the score reels when the buzzing starts? Can you see any of the score reels trying to advance?