The low-frequency dynamics of multimode class-B lasers was studied theoretically and experimentally for a Nd3+-doped glass chip laser. Resonance peaks in the fluctuation spectra of the individual modes and of the total output as well as correlations of various laser modal fluctuations were determined and compared to the results for lasers with only a few modes. Numerical simulations reproduced the observed fluctuations of emission in the laser modes in the cw regime.