  Resolvent analysis is performed to identify the origin of two-dimensional
transonic buffet over an airfoil. The base flow for the resolvent analysis is
the time-averaged flow over a NACA 0012 airfoil at a chord-based Reynolds
number of 2000 and a free-stream Mach number of 0.85. We reveal that the
mechanism of buffet is buried underneath the global low-Reynolds-number flow
physics. At this low Reynolds number, the dominant flow feature is the von
Karman shedding. However, we show that with the appropriate forcing input,
buffet can appear even at a Reynolds number that is much lower than what is
traditionally associated with transonic buffet. The source of buffet is
identified to be at the shock foot from the windowed resolvent analysis, which
is validated by companion simulations using sustained forcing inputs based on
resolvent modes. We also comment on the role of perturbations in the vicinity
of the trailing edge. The present study not only provides insights on the
origin of buffet but also serves a building block for low-Reynolds-number
compressible aerodynamics in light of the growing interests in Martian flights.
