Large-eddy simulation is applied to a M = 1.4, under-expanded, isothermal, turbulent jet issuing from a rectangular nozzle of aspect ratio 4:1. To allow for direct comparison with experiment, the nozzle geometry exactly matches that of a nozzle fabricated for laboratory experiments. The entire flow in and around the nozzle is simulated using the CharLES solver, on a unstructured meshes containing up to ≈ 86 million control volumes. The massively parallel code was scaled up to as many as 20,000 processors. The isothermal rectangular jet is observed to exhibit a strong flapping motion in the direction of its minor axis, connected with a strong screech tone of the same frequency. A mesh sensitivity study is performed, and the far-field noise predictions are compared with measurements conducted by Dr. James Bridges at the NASA Glenn Research Center. Mesh isotropy was found to be an important factor in the correct prediction of mid- to high-frequency upstream noise.