I had a similar problem with my Europa a few months back. If the cable going into the bass got bumped or wiggled just right, I'd hear a brief pop/crack.
Usually, I'd chalk that up to a soldering problem in the cable. Fix that up and keep on going. In this case, however, my cable was fine. Turned out I had a worn-out input jack on the Europa.
I couldn't get at the pins that touch the cable inside the jack. I'd hoped to just pry them out a bit to make better contact with the cable. I called Alembic and they recommended simply replacing the jack ($30, I think). Fortunately, Alembic makes this process very simple with the quick disconnect connectors they use in their electronics. No soldering, yahoo!
To test for this on your bass, just strike a note and wiggle your cable near the input jack. If you hear pops and cracks, I'd first check your cable. If that's OK, you might consider replacing the input jack.
Good luck!