  Significant transition strength in light $\alpha$-conjugate nuclei at low
energy, typically below 10 MeV, has been observed in many experiments. In this
work the isoscalar low-energy response of N=Z nuclei is explored using the
Finite Amplitude Method (FAM) based on the microscopic framework of nuclear
energy density functionals. Depending on the multipolarity of the excitation
and the equilibrium deformation of a particular isotope, the low-energy
strength functions display prominent peaks that can be attributed to vibration
of cluster structures: $\alpha$+$^{12}$C+$\alpha$ and $\alpha$+$^{16}$O in
$^{20}$Ne, $^{12}$C+$^{12}$C in $^{24}$Mg, 4$\alpha$+$^{12}$C in $^{28}$Si,
etc. Such cluster excitations are favored in light nuclei with large
deformation.
