The popping sound heard when lighting the gas produced by hydrochloric acid and magnesium is hydrogen.

The reaction between hydrochloric acid (HCl) and magnesium metal (Mg) produces magnesium chloride (MgCl₂) and hydrogen gas (H₂).

Equation:

$$2HCl + Mg → MgCl₂ + H₂$$

Hydrogen is a highly flammable gas, so when it comes into contact with a flame, it ignites, producing a small explosion and a popping sound.