What could be causing microseisms on Mars?
I think that the mechanism suggested in the article fits the bill the best:


  On Earth, microseisms are ubiquitous, caused largely by the sloshing
  of the ocean by storms and tides. Mars, despite the dreams of science
  fiction writers, has no present-day oceans. Instead, this newly
  discovered noise is likely caused by low-frequency pressure waves from
  atmospheric winds that rattle the surface, inducing shallow,
  longer-period waves in the surface, called Rayleigh waves, Lognonné
  said.


See e.g., https://www.lpi.usra.edu/meetings/lpsc2012/pdf/1994.pdf , for a modeling study of the microseismic events on Mars due to wind.