Interaction between an axially streaming electron beam and an axisymmetric TM mode in a periodically corrugated waveguide is analyzed using a linear fluid model. Effect of the perpendic ular perturbation of the beam by a finite magnetic field is considered. There are two mechanisms of Cherenkov interaction between a beam and a TM mode. One is the conventional mechanism attributed to a longitudinal perturbation. The other is attributed to the vertical perturbation of the beam surface and is dominant in the low magnetic field region. The slow cyclotron mode on an axially streaming beam is able to interact with an axisymmetric TM mode only if beam surface modulation is taken into account. The Cherenkov and cyclotron interactions degenerate for zero magnetic fields. This interaction has higher growth rate compared to the conventional Cherenkov interaction in the region of high current and weakly relativistic energy.