Noise caused by thermal magnetization fluctuations in giant magnetoresistive sensors, mainly in the free layer, limits the achievable signal-to-noise ratio and thus the density of magnetic recording systems. Here, we describe a quasi-analytical method to study this noise. First, by discretizing the free layer into sufficiently small rectangular cells, an equilibrium state of the free layer is obtained using a quasi-static energy minimization technique. A linear tensor relation between the magnetization spectra and the thermal fluctuations is found for the two approaches: 1)Landau-Lifshitz-Gilbert-Brown and 2) collective spin wave excitation. In the former, damping and thermal fields are added to each cell and then a Fourier transform is taken to obtain the spectra. For the eigenmode approach, the lossless dynamic equations are diagonalized, yielding the system's intrinsic eigenmodes. Then damping and thermal fluctuations are added to each eigenmode, independently. The sensor playback noise power spectrum density, a measurable quantity, is calculated by summing up the contributions from each eigenmode or cell magnetization. This computationally light treatment yields a clear physical picture of the phenomenon, relating damping, thermal fluctuation, and noise. We describe each step of the calculation procedure in detail and give examples.