Sanivite 3 Bangs when working?

Hi
I have a Sanivite 3 which still pumps fine, but has developed a banging noise for a few seconds when it stops pumping. This seems to be coming from the top of the unit and might be associated with a sort of surging near or in the rubber elbow outlet almost like something bouncing a valve? The unit pumps vertically for around 2 mts and the outlet pipes shake quite violently.
Is this repairable or will I have to buy a new unit?

John
January 2011