You may be hearing the switching power supply squealing due to a change in it's parameters from a faulty audio output chip or associated component(s). Without a diagram it's difficult to troubleshoot from here. Troubleshooting experience would allow some of us to possibly repair it without a service diagram, but you have the set, and not me or the others here. If you are not experienced in television repair, put the cover back on and forget it... don't risk further damage to it or injury to yourself --- SERIOUSLY!