We measured the modulation index of 0.142 rad when we installed the PMC (See PSL:2251). I checked both PMC loops, none of them are oscillating.

I think I understand now what is happening. The Amplitude modulation from EOM behind the PMC is about -21 dBm . From Evan's earlier calculations , this RF AM should be suppressed by 0.02 only since it is at 14.75 MHz. This corresponds to -34 dB and hence we are seeing a 29.5 MHz amplitude modulaion ahead of about -55 dBm. There is some mechanism which is frequency doubling this modulation and we are seeing it at 29.5 MHz after the PMC.

I think I will try reducing this RFAM with a combination of PBS and tuning both polarizer and EOM angles. But if we had an extra resonant 21.5 MHz EOM and RFPD, this problem would completely go away as PMC will be able to reduce the RFAM more effectively there.