There are three types of sounds. Clicking from the gears/clutch popping, more of a ping type sound from cable threads snapping, and an actual popping sound from the rubber skin snapping back when stretched. The legs do not have cables like the head and tail, so this should not be an issue. The clicking from gears will cause them to wear out more quickly, and thus shorten the life of your pleo, but does not affect the general functioning of your pleo. The popping from the skin is caused by the skin not being properly positioned, so that as the leg bends, the skin stretches like a rubber band until it "pops" back into place. Again, this will eventually cause the skin to stress and break, but in the mean time does not affect he pleo's mobility.
Popping due to the skin occurs once with each leg movement. Clicking from the gears often occurs multiple times per leg movement. If it is the skin, it can often be manually pulled back into proper position and the popping will stop. If it is the gears, and it occurs near the end of the leg movement, sometimes manually moving the leg back into proper position will correct this.
Normally I would always suggest calling customer service about the warranty, but bought second-hand (even if new, never used), it's hard to get warranty support.

But you could try . . .