  We study the `flux noise' spectrum of random-bond quantum Heisenberg spin
systems using a real-space renormalization group (RSRG) procedure that accounts
for both the renormalization of the system Hamiltonian and of a generic probe
that measures the noise. For spin chains, we find that the dynamical structure
factor $S_q(f)$, at finite wave-vector $q$, exhibits a power-law behavior both
at high and low frequencies $f$, with exponents that are connected to one
another and to an anomalous dynamical exponent through relations that differ at
$T = 0$ and $T = \infty$. The low-frequency power-law behavior of the structure
factor is inherited by any generic probe with a finite band-width and is of the
form $1/f^\alpha$ with $0.5 < \alpha < 1$. An analytical calculation of the
structure factor, assuming a limiting distribution of the RG flow parameters
(spin size, length, bond strength) confirms numerical findings. More generally,
we demonstrate that this form of the structure factor, at high temperatures, is
a manifestation of anomalous diffusion which directly follows from a
generalized spin-diffusion propagator. We also argue that $1/f$-noise is
intimately connected to many-body-localization at finite temperatures. In two
dimensions, the RG procedure is less reliable; however, it becomes convergent
for quasi-one-dimensional geometries where we find that one-dimensional
$1/f^\alpha$ behavior is recovered at low frequencies; the latter
configurations are likely representative of paramagnetic spin networks that
produce $1/f^\alpha$ noise in SQUIDs.
