  We study the production of stochastic gravitational wave background from
early dark energy (EDE) model. It is caused by resonant amplification of scalar
field fluctuations, which easily takes place for typical EDE potential based on
the string axion or $\alpha$-attractor model. The resultant spectrum of
gravitational wave background is computed by performing 3D lattice simulations.
We show that, specifically in some class of generalized $\alpha$-attractor EDE
model, a significant amount of gravitational waves can be produced via
tachyonic instability with a peak around femto-Hz frequency range. Models
predicting such gravitational waves can be constrained by the cosmic microwave
background observations.
