  Ruthenium dioxide (RuO$_2$) is an important metal widely used in
nanoelectronic devices. It plays indispensable roles in the applications as
catalyst and supercapacitors. A good understanding of the origin of the flicker
or 1/$f$ noise in RuO$_2$ will advance the design and efficiency of these
applications. We demonstrate in a series of sputtered RuO$_2$ polycrystalline
films that the 1/$f$ noise originates from fluctuating oxygen vacancies which
act as dynamical structural defects, i.e., moving scattering centers. Reducing
the number of oxygen vacancies by adjusting thermal annealing conditions
significantly reduces the noise magnitude $\gamma$, the Hooge parameter. We
quantify the activation energy distribution function, $g(E)$, and calculate the
oxygen vacancy density, $n_{TLS}$, from the measured $\gamma$ value. We show
that $g(E)$ can be explicitly expressed in terms of $\gamma(T)$ and the
electronic parameters of the metal, where $T$ denotes temperature. The inferred
$n_{TLS}$ value is in line with the oxygen content determined from the x-ray
photoelectron spectroscopy studies.
