I've replaced the control arm ball joints, steering rack, and shock absorbers, but the chassis still makes a thumping sound. What could be causing this?

The car runs fine on flat roads but makes noise on rough roads.

I found the reason: the stabilizer bar bushing was worn out. Added a layer of motorcycle inner tube, and it solved the problem perfectly.

Is the steering wheel turned all the way?

Specific problems require specific solutions. The "gudong gudong" noise isn't necessarily from the chassis. Try removing everything from the car (including the spare tire) and then test drive to see if the sound persists.

Did you hit the undercarriage? I deformed the connecting rod under the engine, which caused vibrations. After I replaced it, everything was fine.