We report on the use of ultra-high frequency photoacoustics to detect gratings with linewidths as narrow as 75 nm, buried underneath optically opaque metal layers. Our results show that buried gratings can be detected by observing diffraction from the spatially periodic acoustic replica of the buried grating at the glass/metal interface and from replicas of the acoustic wave inside the glass substrate. The measured diffraction signals show a linear dependence on grating duty cycle rather than the expected quadratic one. We find that this is due to the presence of a coherent background optical field, which interferes with and coherently amplifies the weaker fields diffracted off the grating-shaped acoustic waves. Our measurements show that ultra-high frequency photoacoustics is a promising technique for detection of sub-wavelength periodic nanostructures.