  Evidence for intra-unit-cell (IUC) magnetic order in the pseudogap region of
high-$T_c$ cuprates below a temperature $T^\ast$ is found in several studies,
but NMR and $\mu$SR experiments do not observe the expected static local
magnetic fields. It has been noted, however, that such fields could be averaged
by fluctuations. Our measurements of muon spin relaxation rates in single
crystals of YBa$_2$Cu$_3$O$_y$ reveal magnetic fluctuations of the expected
order of magnitude that exhibit critical slowing down at $T^\ast$. These
results are strong evidence for fluctuating IUC magnetic order in the pseudogap
phase.
