I'm pretty sure mine only has a single rubber ring. Not sure about the drop target reset buzz. My guess is what you have could be normal but if you are that worried you could link to a video of your pin.

There are double rings behind the all 3 of the drop target assemblies. Not sure on your "buzz" question but there is more noise from the AC coils than you may be used to with DC coils.

Sounds like your drop target coil cylinder may be getting stuck in the sleeve when activated (no pun intended). A stuck coil will buzz if not allowed to release once energized and eventually burn out. I would suggest check to see if it’s moving freely by manually activating it with the machine turned off. Many times these parts have incorrectly been lubricated. Only parts which are metal-on-metal sound be lubricated.

(1) There should be two sets of rubber around both drop target banks. You can verify this by looking at El Dorado’s flyer. I think the double rubber helps keep the ball from getting stuck or breaking the plastics. On other drop target games with single bands of rubber (like Lucky Hand and Jacks Open) Gottlieb put other braces behind the rubber to provide this protection.

(2) A properly adjusted drop target bank shouldn’t buzz loudly. This tip from Mr. Pinball explains how to properly adjust it for optimal resetting operation:

https://user.xmission.com/~daina/tips/pub/tip0435.html

The bracket that mounts the reset coil to the drop target bank has four screws. Once loosened, you can slide the coil up and down, adjusting the point at which the solenoid plunger hits the coil stop, which is the depth of the reset. If the bracket is down too far and the coil stop doesn’t halt the stroke just right, the reset arm presses needlessly against the bottom of the “fingers” that lift the drop targets. That makes a helluva racket.

Is the reset bank coil buzzing until the score motor cycle stops, like for a full second? I’m guessing that it resets each bank separately and not at the same time. Which one seems too long, or is it both?

I had both an El Dorado and a Solar City in the distant past and I only used single rubber rings, but by golly both flyers do show double rings, darn it. This was before the internet so how would I know? 8D