I did the comparison with all channels on both machines 50 ohms, then 1 MOhm, both of those with DC, AC, grnd coupling with results identical on all configurations. In all cases the inputs were open. I've done factory reset and SPC a couple times and it did alleviate some minor offsets, but the noise remained.
You're right it doesn't seem high and I can live with it. I Just thought it was odd that this otherwise very well cared for scope has this anomaly. Of the dozens of used instruments I've purchased off EBay this is by far in the best physical condition and asking price was too good to pass up.
Also, the noise is not common. It's completely random across all four channels, so I'm thinking it may not be bleeding through from a common source.