The frustrated translation of isolated CO molecules parallel to the surface, or T mode, has been studied on a Cu(110) substrate using helium atom scattering. A small but significant anisotropy in T-mode vibrational frequencies was found for the on-top site species with oscillator frequencies of 3.6 and 3.2 meV along the [1(1) over bar0$] and the [001] directions, respectively. These results are compared with recent results from time-of-flight electron stimulated desorption of ion angular distribution measurements [Ahner et al., Phys. Rev. Lett. 79, 1889 (1997)], where also a significant anisotropy has been observed.