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We investigate the nonlinear effect of a pendulum with the upper end fixed to an elastic rod which is only allowed to vibrate horizontally. The pendulum will start rotating and trace a delicate stationary pattern when released without initial angular momentum. We explain it as amplitude modulation due to nonlinear coupling between the two degrees of freedom. Though the phenomenon of conversion between radial and azimuthal oscillations is common for asymmetric pendulums, nonlinear coupling between the two oscillations is usually overlooked. In this paper, we build a theoretical model and obtain the pendulum's equations of motion. The pendulum's motion patterns are solved numerically and analytically using the method of multiple scales. In the analytical solution, the modulation period not only depends on the dynamical parameters, but also on the pendulum's initial releasing positions, which is a typical nonlinear behavior. The analytical approximate solutions are supported by numerical results. This work provides a good demonstration as well as a research project of nonlinear dynamics on different levels from high school to undergraduate students.