The 2 Youtube videos are very informative. Both are in the language 'Hindi'. I am from India and understand Hindi, let me give a very high level translation.
The 1st video, 'Review' one, points out the same issue. It says, after a year of usage some of the white keys make so much of noise that sometimes that noise dominates over the sound coming out from the speakers. And it says, that's the only and huge issue with Casio CTX series.
The 2nd video, 'Fixing' one, talks about how to fix the issue. The video is from a dealer of keyboards of different brands. He says, its due to structural issues how Casio keyboards are built. A particular side of the white keys touches the internal plastic structure, this is the side which faces the player, and it happens under the black cover (hump) where all the white icons/prints are there. It doesn't happen initially, but starts annoying after some time, and there is a reason behind that. 1) The design issue is anyway there, 2) He says, he found Casio uses poor quality of white grease inside the cover at that part where this friction happens. Over time, it accumulates dirt, and if you are from Tropical region (India) the grease starts melting. And as a result the plastic parts start hitting each other. He suggests Casio to start using better type of grease, Silicon grease. He also says that it can be fixed by opening it up, re-greasing it etc etc. However, he advises users to refrain from doing that unless one knows what he/she doing.
