  We study the transverse electrical conductivity and the corresponding
magnetic noise of a two-dimensional U(1) spin liquid state with a spinon Fermi
surface. We show that in the quasi-static regime these responses have the same
wave-vector dependence as that of a metal but are reduced by a dimensionless
pre-factor controlled by the ratio of orbital diamagnetic susceptibilities of
the spinons and chargons, correcting previous work. We estimate that this
quasi-static regime is comfortably accessed by the typical NV center splittings
of a few GHz and estimate that the expected T1 times for an NV center placed
above candidate materials, such as the organic dmit and ET salts, monolayer
1T-TaS2/Se2, would range from several tens to a few hundred milliseconds.
