Won't the automatic RMS measurement function find the noise? Or if that does not work, what about the standard deviation of the voltage which should produce the same result?
I think the point was to check agdr's assumption that the noise was not random, but that there were some systematic frequency components to it. (Based on over-interpreting the interpolation artefacts he had seen at maximum time resolution.) Hence the FFT to check for the frequency spectrum. If one only wants to quantify the overall noise level, an RMS measurement should be fine.