Jady, just trying to work out in your post what the da da da sound is.
By chance, is this the sound associated with the slam switch closure? I also had an issue with my Skateball (but could have been any of my Bally SS games) where the slam switch sound came on mid play.
Here's a sound clip of what the sound is like. This is from a Hotdoggin', but the same sound applies to the Skateball. At the 35 second mark of the video, the slam switch is manually activated.
http://www.youtube.com/watch?v=1dYxxZWH1vEIf this is the sound you are hearing, then my gut feeling is you may have a switch closing and not gapped enough. Then again, I may be on the totally wrong direction.