Entrainment and periodic pulling of an ion acoustic instability have been observed in the power spectra of a low-pressure high-beta plasma. The observed nonlinear phenomena can be modeled by using the van der Pol equation with a forcing term. Experimental results of the nonlinear processes are presented. Ion density fluctuations are detected on a negatively biased Langmuir probe for magnetic fields and input powers above 30 G and 900 W at 7.2 MHz respectively, and gas pressure below 1.5 mTorr. This low-frequency instability is observed in the central plasma blue core argon II emission and can be controlled by amplitude modulation of the radio frequency input power at frequencies close to the instability frequency. © 2009 American Institute of Physics.