Signals of period around 6-year were recently observed in GNSS observations, and tentatively attributed to the dynamics of the fluid core. Fluid core motions may indeed induce changes in the fluid pressure acting at the core boundaries, and thus in the Earth's surface topography when considering an elastic mantle. However, we find unlikely the possibility for a O(1) mm interannual deformation at the Earths surface due to core flows, based on the literature. We perform a spectral as well as a spherical harmonic analysis of more than 18 years of GNSS observations and confirm the existence of the 6-year signal previously detected. We also find a significant amplitude of hydrological loading effects at such periods. These should be considered when interpreting interannual deformations.