There is an oscillation in the Xarm at 631Hz, which is not in the Yarm. There is a small peak in POY11_I at this frequency, but only when the Xarm is locked. If the Xarm unlocks, the peak disappears from POY. The peak is 3 orders of magnitude larger in POX than in POY, and 4 orders of magnitude larger than the POY noise when this peak is not present. In the plot, I have turned off the POY whitening, so that its situation is the same as POX (we still need to fix POX whitening switching). Dark noise (MC unlocked) is the same for both PDs.