  We present results from a full general relativistic three-dimensional
hydrodynamics simulation of rapidly rotating core-collapse of a 70 M$_{\odot}$
star with three-flavor spectral neutrino transport. We find a strong
gravitational wave (GW) emission that originates from the growth of the one-
and two-armed spiral waves extending from the nascent proto-neutron star (PNS).
The GW spectrogram shows several unique features that are produced by the
non-axisymmetric instabilities. After bounce, the spectrogram first shows a
transient quasi-periodic time modulation at $\sim$ 450 Hz. In the second active
phase, it again shows the quasi-periodic modulation but with the peak frequency
increasing with time, which continues until the final simulation time. From our
detailed analysis, such features can be well explained by a combination of the
so-called low-$T/|W|$ instability and the PNS core contraction.
