Getting intimate with Sterling!

[Leathertan]

Hi everyone. Long time viewer, first time poster. After plenty of other times reading up on basses and amps and all your associated opinions I wonder if anyone has any advice for me here.
Just got myself a lovely Musicman Sterling (standard 4H model) and after checking behind the electronics plate (nothing), the battery box (nothing), and scratch plate (nothing, it's just flat wood!) I STILL have a random 'knocking' coming from somewhere inside the body, roughly around/under the pickup. It sounds as though the loose moving part is rolling back and forth as you tilt it (imagine holding your bass flat on its back, horizontal and strings up with the headstock to your left and the rear strap-button in your right hand, then tilt/rock it towards you then away from you...!)
Now, I'm no guitar tech and I'm figuring the strings then pickup need removing but really I'm wondering if there's anyone who's had one of these apart or knows the parts involved in them and if such a problem either:
Means the pickup is f***ed and that noise is a death sentence, or
There's a thing that often can come loose and it's easy to whip back together.
It's a fleabay purchase and the vendor has been super cool, even offering a refund if it's no dice.
Thanks in advance for any help, guys and gals!
Dan.