Very-very loud krrr-krrr-krrr-krrr-krrr sound from dc-in

I hear an extremly loud neverending krr-krr-krr-krr-krr-krr-krr-krr sound from the dc-in when i power on the machine (even on battery power if adapter is not connecter).

I guess it is not from fan (its never stops) and not from speaker(there is the same when i take down the volume) becouse the sound is coming from the gap behind the dc-in connector.

When the power is off and I loading the battery (dc-in connected) there is _no_ sound.

Only when i power on the machine.