I am using the M9 and have heard this in some songs when using Hi-Res output (through wired headset/AUX) with the sample rate setting at 192KHz. If I set that lower to 96KHz and I don't hear the crackling anymore.
That leads me to wonder:
1 - Is this crackling introduced somewhere in the re-sampling?
2 - Why is there no option for the sample rate to match the file?