View source for User:Piccobello/Hissing sound on battery

The problem is described [[Problem with high pitch noises|here]]. 
On my {{X31}} I identified two distinct hissing sounds:

== Sound related to screen brightness ==

It took me a while to realize this.. the sound seemed to not depend on CPU
in any way: it would always start when I unplugged the power. I then realized
that the sound would stop when the screen was off, and that I could start it 
while on AC power by simply decreasing screen brightness. 

Only at full screen brightness the sound would stop. There is a BIOS
setting that regulates that: simply switching Config>Display>Brightness from
Normal to High solved  the problem.

This means that my batteries will last less, but if I'm in a noisy environment I can still decrease brightness manually.

==Sound related to CPU==

I noticed another much weaker sound that seems to be turned on whenever I switch
to the performance governor, and is only turned off if I switch to powersave, 
regardless of wether the battery is used.
This does not affect me as I never use the performance governor.