one channel with hum

sometimes I turn them on and one channel has a hum. I'm an electrical engineer, so I attempt a repair. The problem is not a wire, since no movement on the wires changes the result... possibly an intermittent solder connection or component. I opened the unit. The unit is hand made, uses two 8-pin sot ICs and lots of smt 0805 type resistors. There is flux everywhere, but I could not find a bad solder connection using a 8X loop for the eye. The microphone sensor for that channel seems to be the issue, as when I probe that area of the input to the chip... I get a high pitch tone. I do NOT get that result on the coressponding point on the opposite channel. I'll lift the two sence microphone wires and compare. Interestingly, if I touch the negitive return on the battery, or almost anywhere on the board, the noise goes away. Thats why I thought is may be a shield wire open, maybe from the headphone's noise sense microphone. I'll spend another hour, then just buy another pair. other than this "technical failure" they did work great. It's worth a second purchase to try again. Wish I had the schematic. Any one have it... send to ab2kc@optonline.net At least I'll have one for spare parts!!!
Thanks.George