If your vehicle is showing rattling noise in neutral that stops when clutch is pressed, here are the most likely causes and the parts you'll need to fix each one.
The transmission input shaft bearing spins whenever the engine is running and the clutch pedal is released in neutral. When worn, it produces a rattling or growling noise that goes completely silent when you press the clutch pedal to the floor, which stops the shaft from spinning.
Without enough fluid to properly lubricate and cushion the transmission internals, the gears can chatter or rattle against each other while idling in neutral. Pressing the clutch stops the gears from turning, instantly eliminating the noise.
The dampening springs inside the center of the clutch friction disc can weaken, shrink, or break over time. When idling in neutral, these loose springs rattle in their housings. Pressing the clutch changes the load on the disc, stopping the rattle.
While a failing release (throwout) bearing usually squeals when the pedal is pressed, a loose or worn bearing can rattle against the clutch fork or input shaft sleeve when the pedal is up. Applying light pressure to the pedal stabilizes it and stops the noise.
If the clutch was recently replaced and the pressure plate bolts were not properly torqued or lacked thread locker, the assembly can vibrate and rattle. Pressing the clutch pedal forces the assembly tight against the flywheel, temporarily hiding the noise.