You may have a short in the unit and the noise is the unit trying to power up and then the overcurrent protection intervening. The short could be in the speaker wires and not in the head unit. I'm not a professional sound guy so take this with a grain of salt.

__________________
JSR

1983 300D Turbo
1995 E320 Wagon
2001 MR2 Spyder
2001 Subura WRX Sedan