That may be due to your sliders being seized on the caliper carrier, they should move fairly easily with your hands. I've had this issue before which was causing un even pad wear on the rear.
Not sure if this will be the cause of your knocking tho.
The sliders are a fairly easy fix, remove your caliper carrier 2 x 14mm iirc. Use a good set of grips on the metal bit at the end of the rubber gaiter on the carrier, twist untill it becomes free. This may take some doing depending how seized they are.
When and if you manage to free them removing the sliding pin from the carrier, it will just pull out. Clean that pin up with a wire brush, get some grease on it and put it back into the carrier.