Second a scope would be handy here. One for see the waveform on the speaker an after that you can probe pin 2 on the 12pin ribbon cable to see if noise is present there at turn on. What we are doing here is halving the problem. If it is present here it's further back in the preamp. If it is not here at pin2 it is coming from the output transistor.
Being that problem disappears when an external speaker is connected makes me think load. Check the resistance on the speaker.
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