A strong, naturally-occurring “growl” combustion instability was studied for the case of a lean premixed prevaporized (LPP) combustor that shows great promise in reducing pollutant emissions. Phase-averaged particle image velocimetry (PIV) was applied for the first time to an LPP device to measure phase lags and spatial correlations. The extensive data set includes spatial and temporal correlations between six parameters: combustor pressure, plenum pressure, injection velocity, heat release rate (Rayleigh index), flame liftoff distance and flame centroid. Measured phase angles and time lags are consistent with the MIT model of Ghoniem et al., along with the concept of “equivalence-ratio oscillation” discussed by Lieuwen et al. Frequency and phase data prove that a dual-mode Helmholtz resonance is driven by an equivalence ratio oscillation. One common modeling assumption is shown to be not valid; the length of an attached flame is not what is oscillating; instead the flame base oscillates violently due to periodic liftoff and flashback and this presents modeling challenges. Growl boundaries and the effects of varying some geometric lengths were recorded.