I have read lots of posts that there are audio issues with the Astro Slide. Mine also has the noise (there are two different noises a soft hiss/crackle and then a much louder popping that is repeated). For the popping noise, people had noticed that it stops if you turn off the screen or if you just have it on the lock screen. I think I have found the culprit behind the popping noise - the NFC.
I turned off NFC and the noise is gone, even while using the phone and having audio playing in the background. If I turn NFC on, the popping comes right back. For the Hissing noise I have not figured out what that is about, but it is much quieter than the popping which was driving me insane.