The ghost that carries the "F" note doesn't growl when you tap/click on it; it sounds a sweet "F" note :) Tap/click on him enough times to get the "F" note out of him.
Whew, got it. That was a little obscure, though.
Maybe I should've added a visual cue as well. I'm glad to liked it.
It's always good to see something original and intriguing!
But this was a tad obscure.