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!
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.
There are so many possibilities it’s also impossible to list all of them!