Have recently noticed that when going over rough roads the rear passenger side seems to be quite noisy,almost like a quiet knocking sound.It's not from the inside of the car so I can only assume its something to do with the suspension.Any ideas?

 

Deep,its not your tyres on the wings is it.
If you have 17s or 18s and have the car lowered this is probably it.
If not maybe a weak coil or strut.
Maybe a tie bar bush,toe link bush,rear diff mount or rear roll bar link bush worn??
This might seem technical but these are the various bushes under the back of you car!!
Hope this helps
Cheers
Darren