Recent monthly Fscan spectra show a comb at multiples of 0.996785 Hz, in approximately the region 20-100 Hz. Subsequent investigation shows that it probably appeared between Sept 21 and Sept 24, although the exact date is difficult to tell.

Further details:

• The comb is clearly visible in monthly spectra, which show its appearance in September [figure 1-2].
• It's somewhat visible in weekly spectra (although generally too weak for the combfinder to detect), which shows its appearance during the week of Sept 20 [figures 3-5]
• It's essentially invisible in daily spectra unless you know exactly what to look for. Fortunately, once the exact frequencies are known, the median height of the comb teeth can be tracked. It's not possible to confidently determine a start date, but it's likely after Sept 20 and on or before Sept 24. [figures 6-9]

Note that there was a comb in O3 with spacing 0.996806 Hz (which, on inspection of the O3 spectrum, seems to have a double-peak structure). Although they are very close, the new comb comb does not precisely align with the O3 comb, nor with its second peak.