Cuprate superconductors host a multitude of low-energy optical phonons. Using time- and angle-resolved photoemission spectroscopy, we study coherent phonons in Bi2Sr2Ca0.92Y0.08Cu2O8 +δ . Sub-meV modulations of the electronic band structure are observed at frequencies of 3.94 ±0.01 and 5.59 ±0.06 THz . For the dominant mode at 3.94 THz, the amplitude of the band energy oscillation weakly increases as a function of momentum away from the node. Theoretical calculations allow identifying the observed modes as CuO2 -derived A1 g phonons. The Bi- and Sr-derived A1 g modes which dominate Raman spectra in the relevant frequency range are absent in our measurements. This highlights the mode selectivity for phonons coupled to the near-Fermi-level electrons, which originate from CuO2 planes and dictate thermodynamic properties.