RepairPal Certified shops can diagnose and repair your 1999 Mercury Sable. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
Some-thing is loose.
It is impossible for anyone online to perform the ‘required’ *hands-on* inspection to determine what the noise is.
Have knowledgeable mechanic test drive vehicle so this rattling noise can be experienced first hand.
Good luck!