I was unable to remove a ~32 kHz ringing from the north path lock. I thought possibly that it might have to do with polarisation alignment into the EOM used to actuate in the FSS lock: this in the past had converted phase acutation into amplitude acutation from misalignment. However, the BB-EOM was placed so close to the prior waveplate that it was imposible to get a PBS in to check.  I've moved the BB-EOM 6 cm backward to give space for future diagnostics. Picture before and after attached. It seemed to be aligned ok.

After some realignment down the path I was able to get a lock again,  but there is still ringing, this time at 22 kHz.  The waveform and frequency change with some choices of gains. 

I've attached screen shots of what I seen on the osciloscope, not that that helps much. Not sure if a transfer function would help, if the loop is ringing that doesn't make the measurment easy. This is probably something obvious but I'm leaving it for today.

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FYI side note, there is now an obvious small spray of light that I can see on the CCD cameras, like a flare.  Tuning up the alignment seemed to move it off a bit, I don't remember seeing it there before.  There doesn't seem to be an obvious scattering before the vacuum can.