I've got a bit of knocking from my Giant Anthem. I've tried checking the bolts which I can easily access are tightened (not yet checked those covered by the chainrings).
The chainstay pivot bolt on the non-driveside doesn't seem to tighten (it just spins in either direction, albeit feels quite stiff and gritty). Knackered bolt?
you need an allen key on both sides to tighten that bolt and to stop it spinning
Try taking the chainset off and tightening ALL bolts
If you can feel movement when you wedge your finger between the top of the shock and the rocker arm, then it's the shock bushing that needs replacing
In my experience, the pivot bearings themselves last around 18 months, the bushings around 6 months
Stand behind the bike and gently lift it by the saddle. I you feel a knock it's most likely the shock bushing. Cheap and straightforward to replace with the right drift tool, a vice and a hammer. A shop should do it quite cheaply.
Thanks both. Your description matches the knock I've noticed when lifting the bike by the saddle, so I suspect it may be the shock bushing, although I can't feel any movement there when I wedge my finger in beside the shock bushing, so I'll get the chainset off later to confirm once and for all.