An evacuated dual-nanoweb fibre was observed to self-oscillate when the launched power of single-frequency light at 1.55 µm exceeded several mW. Simultaneously a frequency comb spaced by 5.611 MHz, the self-oscillation frequency, appeared in the transmitted optical spectrum. The underlying mechanism turns out to be stimulated Raman-like scattering (SRLS) by the flexural resonance in one nanoweb, initiated by thermal phonons and providing unprecedentedly high gain (of the order ~10⁵ W^‑1m^‑1). The nanoweb vibration mediating SRLS is almost entirely transverse, so that, close to flexural cut-off, the phonon wavevector can be freely chosen while keeping the frequency fixed. We studied this effect using a novel interferometric scanning technique to probe nanoweb vibrations at high spatial resolution from the side of the fibre, perpendicularly to its axis. After these highly detailed experimental measurements, an extensive theory showed that self-oscillation is the result of stimulated intermodal scattering (SIMS) unbalancing SRLS gain suppression [Koehler (2016)]. This novel mechanism allows the design of single-pass optomechanical resonators requiring only a few mW pump power, without electronics or any optical resonator. The design could also be implemented in silicon or any other suitable material.