I just got a brand new Clavinova CLP 645 and I couldn't help but notice from the first touch how noisy the keyboard is. It's not just the thud when the keys reach the bottom, but there is also a clicking/squeaking sound that goes with it which is incredibly annoying and distracting when playing. Did I get a defective copy?
I think I can categorically state you have a defective unit, and should return it. Faulty keyboards are not uncommon.
As support for this answer, as well as personal experience I can cite two replies describing similar situations, and one that confirms your keyboard WAS faulty. All, however, were deleted as irrelevant.