We investigated voxel-wise spectrograms derived from the time-varying spectral patterns of BOLD signals in white matter. Finite-window spectra are non-stationary but may be categorized into five distinct modes that recur over time. Close scrutiny of the signal profiles reveals distinct spatial distributions of the occurrences and durations of these modes, and such distributions are highly consistent across individuals. In addition, two communities of white matter voxels may be identified according to a hierarchical arrangement of transitions among modes. Our findings reveal the non-stationary nature of BOLD spectral patterns, and provide a novel spatial-temporal-frequency characterization of resting-state signals in white matter.