That system has problems with electrolytic capacitors that dry out (and sometimes physically leak). It may not be the CD but the tuner module that is bad (mounted in the trunk with amps). Some time ago I repaired one for a friend that would pop when the volume was adjusted on some functions (it didn't have a CD). Turned out to have quite a few bad capacitors in the tuner module. Replaced all bad ones and the quality of the audio improved dramatically as well as no more pop.