There is likely to always be some ripple/noise on the input and this is particularly noticeable with a low mV scale and a floating input signal. Much of this is eliminated when you measure a low impedance source.
In firmware versions prior to V3.4, a pull-up resistor was (incorrectly) active on V2 hardware ADC input and so would limit floating noise. Re-enabling this pull-up however is not a good option, because the pull-up resistance interacts with source input. For a low impedance load this is less of an issue, but for moderate to high impedance loads, this may cause highly inaccurate measurements.