Knocking/squeaking from the back of my Fabia

Recently noticed what sounds like a knocking noise from the back of the Fabia from inside, and a squeaking noise from outside (sticking the head out the window). It's noticeable at low speeds and/or going over bad roads. Speed humps are great for bringing the best out in it. Never been there before, and I'm wondering if it's one of those "fix now or it'll cost you" type jobs.

Having had a look through the forums, noticed people reporting knocking from the front, not really found anything about back end.

Any ideas what it might be, and how easy it would be to replace? My "skill level" is low level bodger, but with a mate who loves to tinker (when he's not getting Rover 400s to drop on his thumb). Last thing I want to do is to take it to a garage and say "there's a funny noise at the back..." as I swear that lights up the cash light in their eyes sometimes.