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The ability to internally generate and maintain a rhythmic pulse, i.e., beat imagery, is a fundamental aspect of musical cognition. While recent theories propose that premotor regions support internal temporal predictions during rhythm perception and imagery, direct causal evidence remains limited. In this study, we investigated the specific contributions of the dorsal premotor cortex (dPMC) and the supplementary motor area (SMA) to beat imagery using transcranial magnetic stimulation (TMS). Forty-two non-musicians listened to rhythmic musical excerpts and judged whether a probe tone, presented after a short silent period, was temporally aligned with the imagined beat. TMS (three pulses at 10 Hz) was delivered over dPMC, SMA, or a sham control site (coil tilted 90° over M1) immediately before the imagery phase. Participants also completed the Bucknell Auditory Imagery Scale (BAIS) to assess individual differences in auditory imagery abilities. Results showed that TMS over the dPMC significantly modulated beat imagery performance, particularly in individuals with lower auditory imagery scores. No effects were observed following SMA stimulation. These findings provide causal evidence for the involvement of the dPMC in the endogenous generation of rhythmic structure and suggest a functional dissociation between motor-related areas in beat-based timing. Moreover, the interaction between stimulation effects and individual imagery abilities indicates that the neural response to TMS is shaped by individual functional states. Collectively, these results highlight the flexible and context-dependent nature of rhythm imagery mechanisms and support a predictive role for the dPMC, and more broadly, the dorsal auditory stream, in internally guided beat processing.