This paper presents 1/f noise measurements on Si d-doped GaAs structures. The samples are characterized by Hall, magnetoresistance, and Schubnikov–de Haas measurements. The distribution of electrons over the two lowest subbands in these structures varies with temperature and illumination, and so does the noise. The 1/f noise is characterized by the usual parameter a. We show in detail how to interpret the 1/f noise in the two-subbands system. We find that a increases by a factor of 30 upon population of a second subband either by illuminating the sample or by raising the temperature to 100 K. This strong increase in the 1/f noise is successfully described by the mobility fluctuation model, where only the lattice scattering contributes to the 1/f noise. The 1/f noise of the electrons in both subbands can be characterized by the same value of aL =0.4, which is strong support for the model.