My fairly new SX-900 is at a local Yamaha Dealer/Repair shop. On another forum, a few members have (had) a noisy "clacking" sound complaint from their black keys. The 'fix' is to apply a special keyboard grease made by Yamaha. Evidently a certain early batch of SX-900's may have had this lack of grease problem. Yamaha, at first seemed to balk at my complaint, and wouldn't fully acknowledge my problem, but perhaps after a few other complaints, has agreed to cover the cost of repair. (It's only 7 months old) I'm sure they want to avoid a recall.

I would have never guessed lack of grease would be a problem in a new keyboard, in fact, I never knew grease was used at all. Upon doing some Internet research, I found out that indeed grease is used to quiet keys on almost all keyboards of all brands.

The shop is waiting for this special grease shipment from Yamaha, and if interested, I'll keep you all updated on the repair results.
(Hopefully not too long, I'm missing this keyboard)