We do this by selecting all (positive and negative) noise peaks above 4 $\sigma$ in the area of interest, after removing the signal that has been reliably detected in the source finding procedure ( see section \[sec:scfinding\]).

Selecting noise peaks above a certain threshold has an important advantage compared to integrating over all voxels within a large area. Bright HI emission as well as diffuse faint HI emission in the data cubes is cleaned by means of clean masks (Paper I). The clean masks do not include the faint noise peaks used for our statistical analysis here, and these are therefore not cleaned. Because the integral over the [**dirty beam**]{} over a large area goes to zero it becomes very difficult to determine fluxes over extended areas without proper deconvolution. This is inherent to the sidelobe structure of the PSF. Any blind measurement of the total flux over a large area is therefore fraught with problems. However, by pre-selecting noise peaks, we analyse only the central part of the PSF and exclude any sidelobes. We are therefore unable to detect the [[H]{}[<span style="font-variant:small-caps;">i</span> ]{}]{}mass of a very extended, smoothly distributed medium, (this is a consequence of using an interferometer with 