1/f noise in hopping conduction: Role of multi-site aggregates
  We propose a mechanism for 1/f-type noise in hopping insulators based on the multi-electron charge redistribution within the specific aggregates of the localized states located in the vicinity of the critical resistors. We predict that the noise with 1/f-type spectrum extends down to practically arbitrarily low frequencies. 
V. I. Kozub, Y. M. Galperin, and V. Vinokur