Why does it do this? inside frame of the digital piano, there are pins that effectively counter weights so that when you press down on the keys, you get the sense that you are pushing real piano keys down, and when you release the key, the counterweight falls back to its original position. Underneath the counterweight is a long stopper pad, or what I call a felt strip, that deadens the sound when the keys are released. Over time through use, and just simple gravity when not used, this felt strip begins to flatten and the effectiveness of the sound deadening begins to fail.
How Do I fix it? There are a couple ways to fix this issue. The recommended way is to take it to the shop and get them to replace the lower stopper pad, which could be expensive depending on your options for keyboard repair shops. They will take it apart, order the parts, and it will feel and sound much like a new keyboard when you get it back. The do it yourself way, which I have seen done - sometimes well and sometimes disastrously, is to use thin weatherstripping under the counterweights. To do this, you would need to buy the thinnest 1/4 inch self adhesive weather stripping and cut it into small pieces to stick on the felt under the keys. If you try to stick them to the side, then over time, they will get pushed down,and the self adhesive will begin to stick to the counterweights and you will have a new problem. Feel free to contact me for additional support on this issue or contact us if you are in the DC Area and need a repair by visiting http://www.pianorentaldc.com/clavinova-repairs.html