Hi - I'm still getting into the mechanics of how my bike works and trying to deal with things as they go wrong, but I've found one problem a bit puzzling to solve. When I ride and hit 3rd/4th gear, I get this clicking which seems to come from the cogs or chain, sometimes just one sound, other times a couple close together. It only occurs in dry conditions. When I turn it upside down and try to have a closer look, it doesn't recreate the noise. I'm confused. What am I missing?

Thanks

Paul