There are many investigations of the total solar irradiance (TSI) variability over the time scales from days to years. Particular emphasis has been placed on the sources of this variability and on the problems concerning stable equipment operation. The investigations made aboard SMM/ACRIM showed a relation between solar variability and various solar activity manifestations. Even then the TSI power spectrum revealed some peculiar periodicity in the range from 9 to 11 days that was not explained and also a well-defined quasi-4-day periodicity (Froelich and Pap, 1989). Their distinctive feature is that they manifest themselves in many processes both on Sun and on Earth. An understanding of their nature is of importance to solar-terrestrial physics. The earlier investigations of Nimbus-7 data pointed out the possible effect of solar sources of unknown nature and also some residual instrumental effects on the behaviour of TSI variations. As a further development of these studies the spectral analysis was made of the TSI variations using the final data set derived over the range from November 1978 to April 1992 aboard Nimbus-7. In our study the data gaps were filled with an artificial noise signal. The autocorrelation function (ACF) was used for detrending and filtering of the data.