Cooper S R56 Rear Brake Noise

[Chefandy]

OK guy please bare with me as new to this, (from UK) need some info on this.
Fitted pair of rear disc/rotors with pads.
Now have a Knocking from the left side rear wheel when under hard braking, noise also happens when applying hand brake. noise gets quicker with speed, sound like something is on the disc. So this what iv done. Swapped the discs over, fitted a second set of pads, fitted new hub bearing to left rear, checked pins are moving in caliper, checked pads are not moving in caliper, checked both discs for uneven machining, both perfect, checked wheel bolts are tight.
Any ideas/thoughts would be appreciated on this as its giving me brain ache.

Regards to all.