High-fidelity large eddy simulation (LES) is used to investigate the source of crackle noise generated heated supersonic jets. Crackle is associated with intermittent N-shaped acoustics waves consisting of a sudden compression followed by a more gradual expansion, and is extremely irritating to human auditory perception. Results from four different simulations of jets produced by an experimental military-style nozzle reveal that N-shaped waves are generated directly by the jet turbulence, and thus nonlinear propagation effects are not a necessary component of their formation. Different operating points for the simulations were chosen such that jet velocity and jet temperature were varied independently. While crackle levels are sensitive to jet velocity, full field pressure skewness results suggest that increasing temperature may also play an independent role in enhancing crackle. The structure of the crackle source in the most strongly crackling jet is investigated by a backtracking algorithm applied to the simulation resulting in conditional averages.