Two, previously unobserved, low-frequency vibrational modes of c(4x 2) CO/Pt(111), measured using high-resolution helium atom scattering, are reported. The modes, at frequencies of (h) over bar omega = 8 meV and (h) over bar omega = 10 meV, are nearly dispersionless, similar to the previously known low-frequency mode at (h) over bar omega = 7 meV, which was also observed in the present measurements. From the intensity dependence on wave vector and relative intensity to the (h) over bar omega approximate to 7 meV mode, we suggest that the new modes are also polarized parallel to the Pt(lll) surface and, further, that they are associated with the bridge bonded CO molecules.