Why do we hear a pop sound when a burning candle is brought near the mouth of the test tube containing Aluminium and sulphuric acid?

Metals liberate hydrogen gas when treated with acid by displacing the hydrogen of the acid and thus also forming a salt along with hydrogen.

$\mathrm{Metal} + \mathrm{Acid} \to \mathrm{Salt} + {\mathrm{H}}_2$

Thus,

$2\mathrm{Al} \left(\mathrm{s}\right) + 3{\mathrm{H}}_2{\mathrm{SO}}_4 \left(\mathrm{aq}\right)\to {\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3 \left(\mathrm{aq}\right) + 3{\mathrm{H}}_2 \left(\mathrm{g}\right)$

When a burning candle is brought near the mouth of the test tube, a pop sound is heard due to this hydrogen gas.