If the boots are broken, replace the axles. Torn boot allows dirt to enter the joint and wears it, causing binding inside the joint, which is the noise you hear. Normally when a CV joint is failing, it only pops when driving while turning, but I didn't see that condition mentioned. Aftermarket axles are not much more money than a boot kit. Besides, once the joint parts wear, they have to be replaced anyway so you might as well get the entire axle.