  We present a temperature-dependent resonant elastic soft x-ray scattering
(REXS) study of the metal-insulator transition in Sr3(Ru1-xMnx)2O7, performed
at both Ru and Mn L-edges. Resonant magnetic superstructure reflections, which
indicate an incipient instability of the parent compound, are detected below
the transition. Based on modelling of the REXS intensity from randomly
distributed Mn impurities, we establish the inhomogeneous nature of the
metal-insulator transition, with an effective percolation threshold
corresponding to an anomalously low x<0.05 Mn substitution.
