To me it seems like a popping noise caused by a piece of skin would come AFTER the shed. In that situation most of the skin comes off, but a stuck piece is flapping in the nostrils, causing a pop. Before the shed the skin is completely intact, so I don't see how a piece of skin could be causing the sound. No shed has occurred, so why would there be a loose piece of skin flapping?

I could be wrong though.