since some days, we observe the cavity is difficult to lock and a strange 30Hz noise has appeared on the PZT signal which normally compensate the phase noise difference between the cavity and the laser.
to test if the problem could come from the laser, we changed the OneFive laser for the Koheras but we have exactly the same problem, thus we concluded that the problem come from the cavity or from the feedback.

today, we borrowed an accelerometer measurement setup to Julien Bonis to test if we can see a clear noise at 30Hz from the seismic noise.
we placed to accelerometer directly on the top of the cavity but the spectrum we obtained do not show a clear noise line at 30Hz, only a small excess of noise in this region...
nothing which clearly indicate the cause of our problem.

yesterday, we changed the feedback setup in changing the PDH box from n°2 to N°3 without any change in the 30Hz noise line.
today, we also changed the feedback setup by introducing and amplifier of 100 just after the PDH box.
if noise is coupled after the amp we should be more immunized from it... but nothing changed again.

still looking for the origin of this problem...