what causes new ceramic brake pads to work fine for a few days then start making a thumping or knocking noise?

Have you used the brakes hard and damaged the rotors?

Ceramic pads will handle the heat better, but then the discs can be cooked.

look, you can't install ceramic brake pads without using specialty hardened rotors. The soft rotors will immediately develop heat checks and start to pulsate. If you want Brembo quality brakes, you have to install all of the quality parts.

You don't get NASCAR quality brakes by installing pads that the factory rotors can't handle or failure is soon to come.

Hi although the pads have been fitted what about the state of the wheel bearings which is proabably where the noise comes from.