Ok, with the machine powered, I plugged in the Coin unit. No clicks or buzzes, unless they are very quiet and I can't hear them over the compressor running.
You'll hear them, or better yet, you can see them activate if you remove the rejector from the mech and then plug it in. It's the one or two metal bars with fingers protruding into the rejector.
Oops, I see that was mentioned above. Likewise, if the magnets are energized, these can be humming, and can be pulled out from the electromagnets. If the magnets aren't energized, it may actually be the coin mech. These used to go bad frequently on the route. If you can arrange to borrow a verified working coin mech, try plugging that in and see what happens.