  Experimental and theoretical evidence is presented for new low-magnetic-field
($B<5$ kG) 1/B-oscillations in the thermoelectric power of a high-mobility
GaAs/AlGaAs two-dimensional (2D) electron gas. The oscillations result from
inter-Landau-Level resonances of acoustic phonons carrying a momentum equal to
twice the Fermi wavenumber at $B = 0$. Numerical calculations show that both 3D
and 2D phonons can contribute to this effect.
