Direct numerical simulation of a confined laminar premixed flame has been performed in an anechoic combustor, showing self-sustained intrinsic thermoacoustic oscillations. Theoretical predictions based on acoustic jump relations and the n − τ model for the flame are presented and causality concerns are discussed. The instability’s frequency and mode structure are recovered numerically with very good accuracy. A detailed discussion on the interplay of the physical phenomena responsible for this dynamical coupling is also carried out.