I 've just received a free intel 815 motherboard with onboard sound.
The board is good except there is buzzing sound(sound like a machine
gun fires the bullets all the time) from the speaker jack(connect to
speaker). The sound starts at the POST and does not stop at all.

I swap the speaker, the power supply, reinstall all the compontents
and put the board on phone board. The problem still persists.

I have couples of sound cards, so putting a new card is not a problem.

I'm just curious of what part of the circuit is causing the problem?