This paper deals with dynamic analysis of straightening machine loading during straightening process. During it, a crooked bar passing through the machine and the rollers and holders are subjected to the dynamic loadings. The roller's moment amplitudes depend, among others, on straightening speed, size of the bar curvature, bar's diameter, and on friction between the bar and the rollers. The loading is time-varying and knowledge of the frequencies of the moment is important; it can be used for verification of the numerical simulation or for the straightening machine design. Machine operation close to resonance state can cause excessive vibration which can subsequently cause noise and adversely affect the life of the machine and also straightening accuracy. This paper is aimed to analyze frequency spectra of the moments, which are obtained from the computational simulation and find the possible causes of vibration. Although, the straightening process is complex, it was possible to identify three main causes of vibration which are presented as the results of this paper.