If your vehicle is showing rattling or loose noise from inside the steering wheel horn pad, here are the most likely causes and the parts you'll need to fix each one.
The bolts or retaining springs holding the airbag and horn pad to the steering wheel were not fully tightened or seated properly during previous steering column or clockspring repairs.
The wires for the horn, airbag, or steering wheel controls have come loose from their internal retaining clips and are tapping against the plastic housing over bumps.
The internal springs that keep the horn pad pushed outward have weakened or broken over time, allowing the pad to vibrate and rattle against the electrical contacts.
The main center nut securing the steering wheel to the steering column splines is loose. While it sounds like it is inside the pad, the entire wheel hub is actually shifting.