Hey Andrew, thanks for the response, it so happened that I posted the same video asking the same questions on both FB and this forum at the same time. I did try out blowing below the keys with a canister of compressed air and the frequency of occurrence of the problem has reduced. But still it persists and I do not know when one of those black keys will give out a re-sound again if pressed too hard. So I guess authorized service is my best bet for now