GRS 1915+105 is a widely studied black hole binary, well known because of its extremely fast and complex variability. Flaring periods of high variability alternate with ‘stable’ phases (the plateaux) when the flux is low, the spectra are hard and the timing properties of the source are similar to those of a number of black hole candidates in the hard spectral state. In the plateaux the power density spectra (PDS) are dominated by a low-frequency quasi-periodic oscillation (LFQPO) superposed on to a band-limited noise continuum and accompanied by at least one harmonic. In this paper we focus on three plateaux, presenting the analysis of the PDS and in particular of the LFQPO and its harmonics. While plotting the LFQPO and all the harmonics together on a frequency–width plane, we found the presence of a positive trend of broadening when the frequency increases. This trend can shed light on the nature of the harmonic content of the LFQPO and challenges the usual interpretation of these timing features.