The goal of this paper is to characterise the variability of two important design parameters of thermoplastic honeycomb sandwich beams from the analysis of experimentally determined resonance frequencies and mode shapes of a limited number of test beams, under free boundary conditions. The design parameters are the Young’s modulus of the skin in length direction of the beam and the out – of – plane shear modulus of the honeycomb core. These two independent parameters are expressed as random fields using a Karhunen - Loève series expansion of the covariance matrix, available after updating the Finite Element models using the experimental results. The random variables of this series expansion are expressed in terms of a Hermite Polynomial Chaos. The aleatory uncertainty is modelled by the Gaussian random variables while the epistemic uncertainty is described by the randomness of the polynomial chaos coefficients. An estimation of the errors resulting from the experimental and numerical modal analysis is discussed, along with their influence on the two estimated elastic parameters.