Abstract: Primordial fluctuations, generated during inflation, are the seeds of cosmic microwave background anisotropies and the large-scale structure we see today. In the very early universe, small-scale primordial fluctuations are a source of secondary gravitational waves and, if large enough, could also lead to primordial black hole formation. The resulting gravitational wave spectrum depends on the physics of the very early universe. What if modifications of gravity were relevant at the high energies of the very early universe? I will show that the production of secondary gravitational waves is enhanced in Horndeski gravity; the gravitational wave spectrum could be significant even for a small amplitude of primordial fluctuations. I will discuss other interesting issues that appear, such as the gauge and disformal frame dependence of secondary gravitational waves.