When driving down any road the car sounds like something is rattling or lose its not a smooth quiet ride that I would except from a Cadillac. Could it be the Goodyear tires that came on it in stead of Michelin ?
A tire would not rattle. you could have a loose exhaust or heat shield.
also there have been a lot of complaints with owners of your vehicle having a rattle from the dash area.
If it were the tires, why would it take five years to develop a rattle?
As W-W noted, likely exhaust or the under dash as a source.