This is an audio-quality issue.
I was happy with Rockbox since its release for Sansa Clip V2. I was happy until I bought noise-isolating earbuds for my Sansa Clip V2. Particularly at lower volume-levels, now I hear an annoying 4Hz “tick-tick-tick†in the left audio channel, just like a wind-up watch strapped to my ear. I switched back to the OEM Sansa software, and the ticking completely went away, so it's not the earbuds. Last week, I gave up Rockbox for “tickless†audio; however the OEM Sansa navigation and display controls are so bad that I put Rockbox back on after just one day.
Now I can go back to the OEM Sansa earbuds, but I’m sensitized to the ticking sound, and yup, I can just hear the left channel ticking. Because the changelog for v3.9 promised audio improvements, I had hope the update would help, but there’s no difference in my Sansa clip V2 “ticking†problem with 3.9.