If you have an elusive knock/rattle at the back of a Sprint.

[MIG Wielder]

... then try the rear brake balance valve.
There was a posting on here enquiring about this ( can't find it right now ) so if it isn't the usual culprit such damper bushes, trailing arm bushes, upper suspension arm bushes, exhaust etc; etc; then try the balance valve pivot point. Remove the nut and link rod and look for a lot of side-to-side play. It can probably be cured with a new pivot bolt of a new fixing bracket.