  Here we report the spectral-timing results of the black hole X-ray binary 4U
1630-47 during its 2021 outburst using observations from the Hard X-ray
Modulation Telescope. Type-C quasi-periodic oscillations (QPOs) in 1.6--4.2 Hz
and quasi-regular modulation (QRM) near 60 mHz are detected during the
outburst. The mHz QRM has a fractional rms of 10%--16% in the 8--35 keV energy
band with a Q factor (frequency/width) of 2--4. Benefiting from the broad
energy band of hxmt, we study the energy dependence of the 60 mHz QRM in 1--100
keV for the first time. We find that the fractional rms of the mHz QRM
increases with photon energy, while the time lags of the mHz QRM are soft and
decrease with photon energy. Fast recurrence of the mHz QRM, in a timescale of
less than one hour, has been observed during the outburst. During this period,
the corresponding energy spectra moderately change when the source transitions
from the QRM state to the non-QRM state. The QRM phenomena also shows a
dependence with the accretion rate. We suggest that the QRM could be caused by
an unknown accretion instability aroused from the corona.
