Why is a church candle crackling?

A church candle crackling is usually caused by a phenomenon known as "tunneling". This occurs when the wax of the candle is burning unevenly, creating a tunnel-like shape in the center of the wick. As the flame travels down the tunnel, it causes the wax to heat up and expand. This expansion causes the air around the candle to heat up and create pressure, which then causes the wax to crackle and pop. The uneven burning of the wax is usually caused by poor wick positioning, an incorrect wick size, or a draft in the room. To prevent tunneling and the resulting crackling, the wick should be trimmed regularly and positioned properly, and the candle should be kept away from any drafts.