2005 Cadillac STS - Vibration?

I recently purchased a pre-owned 2005 Cadillac STS. The car is in excellent shape and looks and runs great. The guy I got it from had all the service records and all services were performed by the dealer. That said, I've noticed a strange, rythmic type vibration that occurs when I go any where from 60 - 75... show more I recently purchased a pre-owned 2005 Cadillac STS. The car is in excellent shape and looks and runs great. The guy I got it from had all the service records and all services were performed by the dealer. That said, I've noticed a strange, rythmic type vibration that occurs when I go any where from 60 - 75 mph. It kind of goes in an on off pattern about every 1'1/2 seconds. It's not horrible, but seems unusual. I was wondering if this is normal and just a result of the 6 speed auto transmission, or if something else is going on. The car is under extended warranty, but I would like to be able to at least speak intelligently about the problem if I take it to the dealer - any ideas - suggestions?
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