Comment 29 for bug 1477838

"I noticed the same soft white noise continues a second or two after the phone plays any sound from any app, but then turns off. I presume there is some time out turning off some part of the audio subsystem. Presumably if there is a reason, it should happen even when the app causing the sound to play is not active."

So, I believe pulseaudio, by design, maintains active status briefly (ie a second or two) when a connection from an app terminates, so that if two sounds a played back to back there is continutity for the underlying hardware.

I think that's a separate bug (and possibly by design) than an annoying noise happening, which seems to be MX4 specific.

Sadly, so far, finding people with the deep knowledge of the android underpinnings of MX4 is proving tricky.