VOF Sloshing - beating pattern in force output?

Brian

I've been running a lot of 2D sloshing flows (simple translational motion) for a rectangular tank. I've been able to recreate a lot of my experimental results in terms of the global forces in the horizontal direction. However, sometimes I experience a "beating" pattern in my solutions if I force it away from the theoretical resonance. An FFT of the force time history shows one frequency at the theoretical natural frequency of the system, and the other frequency is my forcing frequency inputted to the code. This does not show up in my experiments and I find it to be a bit frustrating. Has anyone else experienced this or know how to get rid of this numerical instability? Thanks!!