While fluid percolates through porous media it generates the acoustic noise in a wide frequency range.
The main reason is that percolation process at pore level is never laminar and micro-turbulence in pore-to-pore fluid migration is accompanied by density variation which creates pressure pulse propagation which is then called the acoustic noise.
The mathematical model of the reservoir noise is given by Reservoir Flow Noise @model.
See also
Physics / Mechanics / Continuum mechanics / Fluid Mechanics / Fluid Dynamics / Fluid Flow / Percolation
[ Reservoir Flow Noise @model ]