Noise diagnostics was performed on a tungsten hairpin cathode that was used in conventional scanning electron microscope (SEM) which operates in a high vacuum. The focused beam was firstly measured and its power spectrum obtained in order to characterize its slope in the lower frequencies which are connected with the events occurring on the cathode surface during the emission of electrons. Further experiments involved additional noise measurements which evaluated electron beam with altering beam energy, in particular at 5 kV, 10 kV and 20 kV up to 30 kV; with and without electron beam scanning involved and with different levels of cathode heating. Obtained results were evaluated in relation to a 1/f type noise component, generation-recombination process on the cathode surface, on the shot noise and on the velocity fluctuations caused by the ion oscillations. Achieved results were discussed.