What could be causing microseisms on Mars? The anouncement published in Science Mini tremors detected on Mars for first time says that the Mars lander InSight is detecting microseisms on Mars after various vibrations produced by the lander and its interaction with the wind are excluded.

The above linked Wikipedia article says:


  In seismology, a microseism is defined as a faint earth tremor caused by natural phenomena. Sometimes referred to as a "hum", it should not be confused with the anomalous acoustic phenomenon of the same name. The term is most commonly used to refer to the dominant background seismic and electromagnetic noise signals on Earth, which are caused by water waves in the oceans and lakes. Characteristics of microseism are discussed by Bhatt. Because the ocean wave oscillations are statistically homogenous over several hours, the microseism signal is a long-continuing oscillation of the ground. The most energetic seismic waves that make up the microseismic field are Rayleigh waves, but Love waves can make up a significant fraction of the wave field, and body waves are also easily detected with arrays. Because the conversion from the ocean waves to the seismic waves is very weak, the amplitude of ground motions associated to microseisms does not generally exceed 10 micrometers. (several citations available in original article)


We can exclude large surface bodies of water on Mars, so the vibrations must be coming from other sources.

Question: 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.