Toroidally symmetric, narrow band low frequency (<2 kHz) MHD activity at the plasma edge has been identified during initial phases of "dithering" LH transitions in JET. It manifests itself as regular bursts on the divertor Dα emission, and is also observed on magnetic and target Langmuir probes and on edge channels of the reflectometer. The oscillations appear simultaneously with the transition from L to the "dithering" Hmode phase and disappear with the transition to the type III ELMy Hmode phase of the discharge. In spite of being regular and in many respects similar to "dithering cycles" previously observed in ASDEX Upgrade and attributed to repetitive series of LHL transitions, these oscillations show clear signatures of ELMs and can be gradually converted into irregular type III ELMs with the increase in edge electron temperature.