1. suspension device problem: the suspension device is a component connecting the cockpit and the frame. If these parts are loose or worn, they may make a "jingle" sound.
2. Slipping: When driving, if the bottom of the cockpit touches stones or other obstacles on the road, it may make a "plop".
3. Component damage: If some parts and fixtures under the cockpit are damaged or loose, a "pop" sound may also be emitted.
4. Chassis brake problem: Failure of chassis brake may also cause this sound. If a component of the braking system is damaged, some components may vibrate and make a squeak or thump.