Keys that make slight thumping sound when they go back up , how do I fix it ?

Hey guys ! I have a Casio celviano digital piano, I’ve had it for about 9 years I love my piano but I noticed the keys make a sound when they go back up, When I press them I understand, but when they click back into place it makes a sound and it’s really annoying me ever since I noticed! Is there a way to fix it ?