I'm posting an easy solution to "clunking", "clattering" keys here in hopes that it will help others with the same problem. Richard Scher's solution above was my inspiration and mine is more aesthetically pleasing if I do say so myself.
My PX350's keys have been "clattering" for some time and got to the point where playing it was no longer enjoyable. The problem so far only affects white keys that are not separated by a black key, in other words the B, C, E, and F keys. As well, the problem I have is only on the upstroke after the key is released.
Instead of using duct tape as in Richard's solution I bought some white foam tape used for household insulation. It's about 1 cm in width and has adhesive backing which I didn't use. I cut a piece of the tape around 8cm in length and had to fold it over 3 times to get the correct thickness. Buying thicker tape or just thicker plain foam would have made it easier. I then pressed the 2 problem white keys down and wedged the foam between the key and the felt as in the picture, similar to what Richard did. The foam stays in place without adhesive!
You can barely see the foam because it's white and it plays pretty much close to perfectly, it doesn't affect the touch of the key and the clattering is gone! The jury's still out on how long it will last, but it's been staying in place and not clattering so far....