Hugh went out and checked all the stops on HAM5 HEPI this morning. Seems there was a stop that got missed that was causing the 1hz peak. After backing the stop off fully, the peak disappeared. 

Attached plot compares L4C RZ spectra (the direction the peak was most pronounced in) from a period last night (purple), when the peak was louder, to a time after relocking this afternoon (green), after Hugh fixed the stop.