If you are talking about the "original manufacture's" 3 year 36k mile bumper and the 7 year 70k powertrain warranties, it transfers with the car. There is nothing to buy. Now, an extended warranty may be a different story.

Back to the original topic... do not let your dealer screw you on the rear end noise issue, it will only get worse over time. Dealers have a trouble shooting guide of known issues. Have your "service manager" type in noise, rear end. It's there, I've seen it.

I had an issue with my service writer telling me the car has moving parts and noise is to be expected. I told him he was an as*hole and a jerk to think I would accept that reply. I went over to another service writer's booth, asked to talk to him, explained my situation with the previous guy and asked if he was willing to work with me in getting my car fixed now and in the future. He apologized for the other guy, looked up the issue and told me it was most likey the rear end. They took my car in, popped off the diff cover and checked for metal shavings. The complete rear end was ordered, received and installed within 7 working days... no more noise.