  We reconsider the gravitational wave spectrum induced by scalar perturbations
in spatially flat Friedmann-Lema\^itre-Robertson-Walker spacetimes, focusing on
the matter- and $\Lambda$-dominated epochs. During matter domination,
sub-horizon modes are not free and a commonly applied approximation for the
derivative of the tensor perturbation is flawed. We show analytically that this
leads to a significant overestimation of the energy density spectrum. In
addition, we demonstrate that gauge-dependent non-oscillating tensor
perturbations appear in the presence of a cosmological constant. Complementing
the analytical calculations, we compute the according present-day spectrum
numerically for a Planck-like $\Lambda$CDM cosmology, finding that
non-oscillating growing modes appear during the transition between matter and
$\Lambda$ domination in conformal Newtonian gauge.
