  In the presence of random hyperfine fields, the noise spectrum, \delta
s_{\omega}^2, of a spin ensemble represents a narrow peak centered at \omega =0
and a broad "wing" reflecting the distribution of the hyperfine fields. In the
presence of an ac drive, the dynamics of a single spin acquires additional
harmonics at frequencies determined by both, the drive frequency and the local
field. These harmonics are reflected as additional peaks in the noise spectrum.
We study how the ensemble-averaged \delta s_{\omega}^2 evolves with the drive
amplitude, \omega_dr (in the frequency units). Our main finding is that
additional peaks in the spectrum, caused by the drive, remain sharp even when
\omega_dr is much smaller than the typical hyperfine field. The reason is that
the drive affects only the spins for which the local Larmour frequency is close
to the drive frequency. The shape of the low-frequency "Rabi"-peak in \delta
s_{\omega}^2 is universal with both, the position and the width, being of the
order of \omega_dr. When the drive amplitude exceeds the width of the hyperfine
field distribution, the noise spectrum transforms into a set of sharp peaks
centered at harmonics of the drive frequency.
