The low frequency Schottky diode noise has been investigated in GaAs power MESFETs. For those devices, gate noise spectra are generally composed of 1/f and shot noise contributions. We have observed an increase by two orders of magnitude of the noise level when MESFETs are submitted to rf life-test. The increase of the 1/f noise can be explained by a modification of the gate space charge region extension. This interpretation is sustained by a reduction of the drain current transient magnitude and the inherent active trap density. A correlation is assumed between the increase of the shot noise level after rf life-test and a micro-plasma formation. Both 1/f noise and shot noise evolution might originate in a local increase of the electric field in the vicinity of the gate in drain access region. We have demonstrated that LF gate current noise is an early indicator of damage mechanisms occurring at the gate-semiconductor and passivation-semiconductor interfaces of the devices.