Rob French (KC4UPR)
Ah-ha. Two new things... one, this is ringing. I checked the spectrum on the dots a little more carefully, and I can see that those additional spikes are clearly multiples of the peak frequency. So when a signal gets centered up in the filter, it has a tendency to generate the ringing.
Two... this was heavily exacerbated by my AGC. In retrospect, as one would expect (I think), this is is worse with AGC Fast, and better with AGC Slow... with fast, the abrupt change in amplitude is more likely to generate the rapid transitions that causes the harmonics.
That said, even with my AGC turned off, I can still find the harmonics/ringing, they just don't have the pronounced "thump" effect. With AGC Off, they cause more of a buzz/resonance.
Still looking to figure out how to remove them. In retrospect, my three section filter is probably not ideal... I'm considering trying to modify one of the sections to be a low pass filter to try to tamp down the harmonics.