Tapping the surface could just be a long shot means of diagnosis for a CF frame like this at best. You might back it up with a visit to a a shop that is familiar with these Kestrels. These monocoque design CF frames are just a whole different ballgame compared to more conventional lugged or welded frames, so you need people that are used to dealing with this type of bike to look at it too for the best diagnosis.
Just note though, that if there is a crack there, that it is repairable by companies like "Calfee".
I suggest that you don't ride the bike till you get a better idea on what's going on, lest you further damage the frame (and possibly get into an accident too) if it is a crack.

Chombi