Yamaha S80 key ‘clack’

My S80 makes an audible ‘clack’ sound when releasing a black key while depressing an adjacent white. You can feel the black key hitting the white. Happens on less than a dozen keys. Most often when moving from a black to white. I suspect something like a felt strip is worn allowing contact between the two keys but I haven’t opened it up to check.
Has anyone had a similar problem?