Buzzing sound first 30s after speakers turn on. DAC speaker output

Hi,

The other day in the shop I got a macbook pro (non-unibody) with an odd problem. Every time the speakers are toggled by a sound, the speakers give a sort of humming high-pitched for the first 30 seconds.

Googling around a bit gave me the idea this has to do with the settings for the DAC output buffer, where the output only powers down x seconds after the sound was used. I have no clue however how to alter this from 30 seconds to for example 1 second.

Does anyone know how to do this or how to fix it by replacing some hardware or reseating something? The sound is present both with internal speakers and headphones.