To ensure road safety, periodic monitoring of road surfaces' quality and conditions is necessary. Lately, near-surface geophysical methods have gained prominence in such studies. This article presents field experiments' results that demonstrate the possibilities of cavities detection under a rigid pavement using flexural standing waves. Accumulation of numerous amplitude spectra of acoustic noise records was used to study standing waves arising in the interior of the pavement. It is shown, joint visualization of the averaged spectra obtained by profile observations over a cavity under pavement allows one to identify several flexural standing wave modes by alternation nodes and antinodes along profile. Amplitude distribution map of one of the flexural standing wave modes over a cavity (gully) under pavement was constructed. At a qualitative level, this distribution is consistent with computer simulation results using the finite element method. The fact that flexural standing waves are formed under acoustic noise influence in some part of pavement indicates absence of rigid contact at its lower boundary. Thus, horizontal cavity dimensions can be estimated from the area size on which flexural standing waves are formed.