SO CLOSE! PLEASE, PLEASE! I hope someone can help me on this last bit. It turns out that if I touch the exposed metal end of capacitor C11 on the power supply board and the chassis with same hand, the buzzing goes away completely! Capacitors C2, C4 and C5 had a similar effect but wouldn't kill the buzz completely, just reduced it. None of other capacitors on that board displayed a similar behavior. I tried touching the side of C11 instead of exposed metal end, or pushing on the exposed metal end with a plastic tool instead of my finger, and neither had an effect. I think this would argue against a bad solder joint, but not convinced of my deductive skills anymore. I lifted the board off the plastic mounts to look at solder joints, and in this configuration, touching the exposed metal ends of either C2, C4, C5 or C11 reduced buzz, but didn't eliminate it completely. Same with touching Pin 14 (ground for C2,4,5, and 11). This most likely indicates a grounding problem, but not sure what else to do. I truly hope someone can point me in the right direction, I feel like I almost there!
edit: added a jumper between Pin 14 and Chassis. Buzz reduced even with board not on plastic mounts, but now "just" touching the exposed metal end of the capacitors listed above gets rid of buzz, don't have to touch hand to chassis as well