Serial recall of visually presented words is markedly impaired when irrelevant speech is presented. Using EEG coherence, a previous experiment investigated the underlying phonological rehearsal network and found coherence changes as a function of rehearsal processes (quiet vs. irrelevant speech condition) at central and left frontal electrode sites in the Gamma frequency band. In the current study we varied the recall modality of a delayed serial recall paradigm to test whether participants show similar synchronization patterns when being forced to encode the visual items into a phonological code (spoken recall) or not being forced to do that (written recall). The duration of high EEG coherence during the retention phase revealed different patterns for different frequency bands: In the Gamma band, the spoken conditions showed the previously observed decrease of coherence duration from quiet to irrelevant speech in central and frontal electrode combinations whereas the written conditions did not. In contrast, Theta coherence showed an increase for the speech compared to the quiet condition in fronto-parietal electrode combinations only in the memory tasks with written but not in those with spoken recall. The results are interpreted as indicating that differences exist concerning the networks involved in the rehearsal mechanisms employed in the two different tasks.