Source of 5.5 Hz Noise showing up on ADC
Aliasing is a distinct possibility. 

If your local AC mains frequency is (say) 51.375 Hz and there are load currents in your house wiring that are harmonically rich, the 4th harmonic would be at a frequency of 205.5 Hz. If this harmonic infiltrates your signal wiring prior to the ADC then you will get 5.5 Hz content in your digital values: -



In the picture above the red waveform can be regarded as picked-up interference at 205.5 Hz. The black dots represent sample points at a rate slightly slower (200 Hz) and the blue line represents the aliased signal in your digital values (5.5 Hz).