  In this work, we study the previously unexplored resonant region of neutrino
self-interactions. Current disagreement on late and early time observations of
the Universe expansion could be solved with new physics acting before the
recombination era. Nonstandard neutrino self-interactions are among the most
appealing candidates to solve this issue since they could be testable in the
near (or midterm) future. We use linear cosmological datasets to test neutrino
self-interactions for a sample of fixed scalar mediator masses in the range
$10^{-2}$ eV $\leq m_{\varphi}\leq 10^{2}$ eV. The resonant behavior produces
observable effects at lower couplings than those reported in the literature for
heavy and light mediators. We observe that in the best case scenario, using the
Planck + BAO dataset, the tension with local measurements of $H_0$ eases from
4.9$\sigma$ (for $\Lambda$CDM) down to 2.8$\sigma$. Albeit, this is driven
mainly by the addition of extra radiation, with $\Delta N_{\rm eff}\sim 0.5$.
The joint dataset which includes Planck, BAO, and $H_0$ prefers a nonzero
interaction from 2.3$\sigma$ to 3.9$\sigma$ significance in the range $0.5$ eV
$\leq m_{\varphi}\leq 10$ eV. Although, this last result is obtained with data
that are still in tension. These results add the last piece in the parameter
space of neutrino self-interactions at the linear perturbation regime.
