I have had the car in several times to replace crank sensors. When only two months old, the lock on the drivers side door came loose and fell inside the door. I have the same problem as several other people with vibration between 60 and 70 mph which I had thought was an unbalanced tire problem. Apparently, it is not as others on this forum have commented about various unsuccessful efforts to fix this same problem in their Deville or Seville.
I had a problem with knocking at about 25000 miles. I took it to the dealer for service and found it was a common problem with this model of Cadillac, and it was apparently caused by carbon build up in the cylinders. He removed spark plugs and filled each cylinder with a cleaner and let it soak for 24 hours. The cleaner was then removed and the spark plugs reinstalled. It did fix the problem, but the service representative acknowledged that this problem might reoccur. I found my own solution to the problem. I started using premium gas instead of the regular gas the manufacturer recommended. Also (and this is very important), I put a bottle of Chevron Techron into the gas tank every 2000 to 3000 miles. I have put 25000 additional mile on the car, and I have had no more problems with the knocking.
I still have the vibration between 60 and 70 mph. I guess I will just have to live with this problem.