We have investigated the spectrally resolved coherent phonon oscillation in semimetals of Sb and Bi. The oscillation initial phase is pgr-shifted for the high- and low-frequency components of the probe pulse, indicating that not only the integrated intensity, but also the spectral content of the probe, are a function of time delay. This means that the coherent phonon, corresponding to nuclear motion, is a wavepacket localized in different regions of the phase space. Furthermore, the spectral filtering results in the appearance of the signal for negative delay time with the simultaneous disappearance of the coherent artefact. We conjecture that all these findings are the indication of the non-classical nature of coherent phonons that has been earlier inferred from squeezing and revivals of the lattice mode in semimetals.