What causes the rattle in a single piston disc brake assembly that disappears when the brakes are applied?

The rattle in a single piston disc brake assembly that disappears when the brakes are applied is primarily caused by loose pads within the caliper. When brake pads are not properly secured or fit too loosely in their designated slots, they can vibrate and create a rattling noise during driving. This noise is typically eliminated when the brakes are applied because the hydraulic pressure forces the pads tightly against the rotor, securing them in place and eliminating movement.

In this scenario, the other factors would generally not cause this specific symptom. For example, while a worn brake rotor can contribute to various brake issues, it typically doesn't lead to a rattle that disappears under braking. Improper caliper alignment may lead to uneven wear or other performance issues but not usually to a rattling sound that resolves with brake application. A defective brake hose could lead to brake performance issues, such as fluid leakage, but it is unrelated to any rattling noise from the pad assembly. Therefore, the loose pads in the caliper are the direct cause of the rattle experienced in this context.