Hello! I just joined this is my first cadillac 2005 STS It had 18798 miles verified through 2 different reporting agencies(carfax & autocheck) The car was pre-owned certified by the dealer and is in great shape, fully loaded. The only problem is that there is a low-speed rumbling from the rear suspension over rough roads or bumps. At high speeds 70mph+ the car is fine. Is this rumbling and occasional thump normal or is there a fix? I would like to get some owner input before I start going to the dealer or independent shop.