Audio buzz / noise in connected HIFI when there should be no audio output

Binary package hint: rhythmbox

9.10 Beta
Alsamixer 0.9.7
Hardware: Conextant CX20561 (Hermosa) IEC958?

When no package is outputting sound, or at startup/shutdown there is an intermittent buzzing sound coming through HIFI system plugged into front headphone jack (of HP DV6000 Series AMD). The sort of static one would usually hear after laptop turns off.
Guess this is Alsamixer problem (but logged here as I couldn't find a 'report problem' for Alsa)
Normally (with last few versions of Ubuntu) the sound would only buzz like this when laptop was off (and presumably the audio line was no longer grounded).
With 9.10 if no app is outputting sound, or sound is paused/stopped in an app (say Rythymbox) the buzz kicks in intermittently.
It's like the driver is leaving noise on the audio channel when it is idle rather than giving a zero/grounded signal.
Doesn't seem to come through headphones, so needs amp in HIFI to bring it up.

Sound management is however much better overall, in terms of simultaneous sound outputs from multiple programs, AND the HP int mic problem has been fixed - yeah!!

ProblemType: Bug
Architecture: i386
Date: Sat Oct 17 22:32:45 2009
DistroRelease: Ubuntu 9.10
ExecutablePath: /usr/bin/rhythmbox
NonfreeKernelModules: nvidia
Package: rhythmbox 0.12.5-0ubuntu2
ProcEnviron:
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcVersionSignature: Ubuntu 2.6.31-13.44-generic
SourcePackage: rhythmbox
Uname: Linux 2.6.31-13-generic i686