squealing/whistling audio

[Benjamin Kaduk]

On Tue, 18 Sep 2012, Doug Barton wrote:

> Sometime in the last couple of months an old problem has resurfaced on
> HEAD, a sort of squealing/whistling sound in the audio, even without
> anything playing. The sound is similar to the wind whistling through
> something.
>
> Before I blindly go off on a bisecting spree, does anyone have a
> suggestion as to where I might look?

It calls to mind this thread:
http://lists.freebsd.org/pipermail/freebsd-current/2009-February/003608.html
(with later followups 
http://lists.freebsd.org/pipermail/freebsd-current/2009-July/009839.html 
and 
http://lists.freebsd.org/pipermail/freebsd-current/2009-July/009850.html 
(and others?))

I had relief in my headphones with machdep.idle=spin, though that is only 
really useful when I am not paying my electricity bill...

I agree with Matt that the noise is physically coming from analog 
leakage/crosstalk, and a software remedy will be something like disabling 
unused lines/pins.

-Ben Kaduk