Put the car up on a lift and find the component that is rattling, start with the stabilizer bar end link bushings, if they are worn to the point that there is metal to metal, then these could be a cause of a rattle over bumps.
RepairPal Certified shops can diagnose and repair your 2000 Volvo S40. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
is this something I can do with the car on jack stands?