Low-frequency noise in metallic single walled carbon nanotubes is shown to be strongly dependent on the Fermi level position and the applied electric field across the nanotube. Resonance-like enhancement observed near optical phonon energy only when the Fermi level lies near the Dirac point is correlated to Raman G-band softening and broadening. The results suggest that the competition between zone-center and zone-boundary phonon scattering is the underlying origin of the large enhancement and resonance-like behavior of 1/f noise.