In the H-T phase diagram of itinerant metamagnets, the first-order transition line terminates at the critical end point, which is analogous to the critical point of the gas-liquid condensation line in the p-T phase diagram. An ultrasonic study of two itinerant electron metamagnets, UTe2 and UCoAl, in magnetic fields was conducted to understand how critical magnetic fluctuations affect their crystal lattices. Sharp anomalies of the elastic constants at the critical field of the metamagnetic transition become maximized in a narrow temperature interval around 10.5 K. This temperature corresponds to the critical end point. We attribute these anomalies to lattice instabilities induced by critical magnetic fluctuations via the strong magnetoelastic interaction. The observed anisotropy of the anomaly is discussed in connection with the anisotropy of magnetic fluctuations.