  We investigate ultra-high frequency gravitational waves (GWs) from gravitons
generated during inflationary reheating. Specifically, we study inflaton
scattering with its decay product, where the couplings involved in this $2 \to
2$ scattering are the same as those in the $1 \to 3$ graviton Bremsstrahlung
process. We compute the graviton production rate via such $2 \to 2$ scattering.
Additionally, we compare the resulting GW spectrum with that from
Bremsstrahlung as well as that from pure $2 \to 2$ inflaton scatterings. For
completeness, the GW spectrum from graviton pair production through one-loop
induced $1 \to 2$ inflaton decay is also analyzed. With a systematic comparison
among the four sources of GWs, we find that $2 \to 2$ inflaton scattering with
its decay product can dominate over Bremsstrahlung if the reheating temperature
is larger than the inflaton mass. Pure inflaton $2 \to 2$ scattering is
typically subdominant compared to Bremsstrahlung except in the high-frequency
tail. The contribution from one-loop induced $1 \to 2$ inflaton decay is shown
to be suppressed compared to Bremsstrahlung and pure inflaton $2 \to 2$
scattering.
