First thing would be to check the connectors//cables - wiggle and see if the problem changes. Remove sound board and check the reverse side at the pins for the cables under magnification. Make sure there are no cracked pins. If no visual indication is found, swap the left and right speaker and test again - if the problem moved (followed the speaker when switched) the problem is a defective speaker. If the problem stays in the left channel, then check output amplifier section for the left channel. Test and replace capacitors on the final output, or possibly final amplifier MB3730 for the left channel. Beyond that, you would likely need an oscilloscope and compare left to right channel to see where the hum is coming from.