An introduction to computer audio
Especially notebooks do use power saving schemas.
A tweak is to disable this option to improve sound quality.
Running on battery power to eliminate the switching power supply is recommended too.
An analog loop back recording (sound card out in to sound card in) with RMAA
This is really horrible. Switching to battery power do gives you a clean DC source but at the same time the power saving kicks in generating a lot of distortion.