In this paper, we describe an implementation of a real-time sound synthesizer using Finite Difference-based simulation of a two-dimensional membrane. Finite Difference (FD) methods can be the basis for physics-based music instrument models that generate realistic audio output. However, such methods are compute-intensive; large simulations cannot run in real time on current CPUs. Many current systems now include powerful Graphics Processing Units (GPUs), which are a good fit for FD methods. We demonstrate that it is possible to use this method to create a usable real-time audio synthesizer.