Low frequency noise measurements have been performed on conventional AlGaAs/GaAs HEMTs in the temperature range-110-300K and in the frequency range 10 Hz-25kHz. The characteristics of deep levels have been determined from the channel noise current spectral intensity evolution versus the temperature. The contribution of the DX centre to the channel noise current has been evaluated for different gate bias. A correlation between the parasitic MESFET operation in the HEMT and the channel noise behaviour has been found.