  This paper reports recent progress of ultrasonic measurements on
URu$_2$Si$_2$, including ultrasonic measurements under hydrostatic pressure, in
pulsed-magnetic fields, and the effect of Rh-substitution. The observed changes
of the elastic responses shed light on the orthorhombic-lattice instability
with $\Gamma_3$-symmetry existing within the hidden order and the hybridized
5$f$-electron states of URu$_2$Si$_2$.
