Filling makes a popping sound. what could this be?

Might be loose: See a local dentist to have it examined and replaced if necessary.

Broken filling: There's probably no need to worry. You should call your dentist and schedule an appointment to have your dentist to evaluate the tooth and replace the filling. If the filling was recently placed, and it came out during the course of normal chewing, then your dentist will replace at no cost. Take care.