I've been gradually having problems with my Keylab 88 MKii keys rattling. I didn't notice it when new, but over time more and more keys were getting noisy, Probably two dozen were bothering me with a rattle.
I contacted Arturia support and they sent me a 15 second video of how to fix it, and now all the keys are quiet!!
Basically, you just have to reseat each affected key. If you press down the adjacent keys with one hand (say B and D if you are fixing C) and with the other hand tightly grip the C key and pull out away from the keyboard (horizontal, not up!). It slips out about an eight or quarter of an inch (but does not come loose entirely). Then you push it back in. And now the rattle is gone. I did this to all my affected keys within about 2 minutes, and it sounds much better now.
I am assuming this is some design issue, and that I will simply have to do this procedure whenever a key gets noisy. It's so quick and easy that I don't expect it to be a problem now that I know. I don't know why this isn't better known.