Insufficient penetration and popping sounds during welding may be caused by:

Insufficient penetration during welding can be influenced significantly by travel speed. When the welding torch moves too quickly across the workpiece, it does not allow enough time for the heat to adequately penetrate the base material. This rapid movement can lead to a shallow weld bead, resulting in inadequate fusion between layers of metal, which is what causes insufficient penetration.

The popping sounds often associated with this phenomenon can arise from the rapid cooling of weld metal, which can occur when the travel speed is excessive. These sounds are indicative of gas entrapment or porosity in the weld, as there isn't enough heat to form a smooth, consistent weld pool that solidifies properly.

Other factors, such as heat input, travel speed, and electrode angle, also impact the welding quality, but in this case, the condition of travel speed directly correlates with both insufficient penetration and the popping sounds experienced during the process.