Hum may be caused by poor contact of signal input line or poor power supply filtering. You can try plugging it into another stereo and check it.

Turn down the volume of the volume potentiometer, that is, mute it. At this time, there should be no sound. If there is buzzing, it is caused by the change of electrolytic capacitor capacity of amplifier filter circuit. If the sound is turned up, there will be a loud buzz, which is caused by the poor shielding of the audio input of the power amplifier. Check and eliminate the shielding failure. The shell of each original factory must be grounded, and the audio input line must be shielded and well grounded.

The hissing sound of the horn is mostly caused by poor overall shielding or poor insulation of the primary winding of the power transformer. The solution is actually very simple, just turn off the power plug. If it doesn't work, it can be eliminated by connecting two 4700P capacitors in series at the power input (the power input of the transformer), connecting the center tap to the shell, or because an audio input is poorly shielded.