When you say multiball, is it truly multiball mode? Does Rudy wake up from sleeping and trap door opens up for multi millions, music starts playing, or does a second ball just come out for play?

If a second ball just comes out for play, the disc that stops the ball at the end of the hidden hallway needs adjustment. If the notch is not right where it needs to be, it can cause the wrong switches to be triggered in the hallway and allow it to release more than one ball.

If multiball mode is actually starting, there is another combination of switches triggering the switch in Rudy's mouth. Probably a backwards diode on one of the other switches, which will have to be diagnosed using the switch matrix.

If it just started doing this, the disc probably just needs adjustment. I do not think switches in the tunnel are causing this as the switch to trigger multiball is in Rudy's mouth. The ball never enters the tunnel from Rudy's mouth. It simply gets spit back out.