Sometimes that "popping" noise is just from the bar tape bending and crinkling as you flex the handlebars. However, it's never a good idea to ignore a potential problem with the handlebars, stem, or fork, so you should check for cracks near where you think the sound is coming from.

Another possibility is that your headset is dirty and the bearings are making a crackling noise when they're under stress. Have you serviced your headset recently? Have you been riding in rain and mud a lot?