I see you've replaced the caliper sliders so this may be irrelevant. I have a similar issue and it is rattling pads on the front brakes (sounds like knocking when inside the car) Simple check, find a nice bumpy road (shouldn't be too difficult!) drive along at normal speed and listen for the noise. Then do the same but GENTLY press the brake pedal with your left foot while driving, you're not looking to slow down just getting the pads to kiss the discs. If it's quieter the noise is probably down to the pads rattling on their pins. Good luck