Procedure

1. Aligned X-arm to IR.
2. Aligned green to the X-arm.
3. PSL green and X-green aligned to the X-green beat PD.
4. Scanned X-green laser temperature (sweep X slow servo offset through the whole range)

I did not succeed in finding the beat note; but noticed something I cannot explain.
With green very stably aligned to the X-arm, GTRX reads 3000 counts. But when the laser temperature is changed and the green unlocks and locks to the X-arm, it locks with GTRX counts over 5000. GTRX stays at 5000 counts as long as the temperature is changing but settles down to 3000 (over a time lapse of tens of seconds) when let to stay at any specific temperature.