Selective electron beam melting (SEBM) is an additive manufacturing process for the production of complex metallic parts. To optimize their mechanical properties, pores and fusion defects, since not completely avoidable, have to be detected and evaluated. The generated microstructure may contain not only voids, but also weak bonds, sharp interfaces and loose particles causing nonlinear elasticity. This paper reports on Nonlinear Impact Modulation Spectroscopy (NIMS) experiments applied to a set of Ti–6Al–4V samples, each of which contains an internal zone with well-defined porosity ranging from 0.2 to 20%, respectively. Resonance spectra, attenuation and indicators of material nonlinearity were evaluated, which allowed the localization, dimensioning and assessment of porous zones. Properties of used linear and nonlinear methods are discussed.