Help with a loud POP

I set up an Airport Express system using the analog out, but decided to try using the digital out to see if an external D/A would improve the sound. Bought a used PS Audio Superlink on eBay and connected it to the optical out on the AE. The PS Audio sounded much better than the internal D/A. Unfortunately, there is a loud "pop" whenever I stop a song or at the end of a playlist. So far, simply turning down the volume (stereo, iTunes) does not affect the pop - only cutting the WiFi connection stops the pop. (There is no pop when using the internal AE D/A.)

Any ideas? Thanks in advance.