Preamp "Snap, Crackle, Pop"

I recently purchased an Antique Sound USA PA-5 preamp from a fellow A-gon user. I have been enjoying it, and love the sound; however, I recently started hearing some disturbing amounts of noise during playback of quite passages. The preamp uses 12AX7s and 12AU7s for each input and a GZ34 for the output. I have tested with another preamp and heard no noise, so I ruled out my Amplifier (Jolida JD-502P). I'm fairly certain the problem exists on the output, because I tested the preamp by switching inputs at full volume (it was only a test!) and heard the same snap, crackle, and pop on each input. When it was sold to me, I was told the phono input had this issue, but since purchasing, each input has developed this disappointing "feature".