Clicking / light knocking from rear when braking

Hi all,

I have noticed that under braking (seems at low speeds only) there is a clicking / knocking noise coming from what appears to be the n/s rear wheel. It's like a heavy click / light knock and not like a scrape. It doesn't do it when not braking.
Could it be anything to do with the handbrake or is it something else?
Other than that the car drives fine and there are no strange noises anywhere else.
Not too sure where to start looking so any advice would be appreciated.
('05 td4)
Thanks.

It could be the brake shoe hold down pin has rusted through. It's also worth checking the wheel nuts are correctly torqued up.