This particular issue has been discussed before in great detail. It's obviously not a good situation for those that have unfortunately purchased a new keyboard with noisy keys. It's probably due to a batch of boards that the grease was not applied correctly, or simply not enough was used
My particular SX-900 was not that "noisy", but I took it to an authorized Yamaha repair shop anyway, just to check it out. The tech said he only noticed a normal click that all keyboards have in various noise levels, and was really unnoticeable at all while actually playing. But said, while it's here, and Yamaha has agreed to apply grease, we might as well do it.
Anyway, Yamaha USA agreed sent him the official factory grease, and I was not charged anything by the shop. At that time, the keyboard was still under warranty. I'm not 100% sure how Yamaha would handle this if the warranty had expired, but my experience with Yamaha (USA) has always been positive.
My advise is to contact Yamaha directly, and explain the problem, then find an authorized Yamaha Repair Shop to fix your problem.
Good Luck