I am having a problems with my Hartke HA7000. The amp makes this random scratchy sounds when it is on. Almost sounds like a bad input jack on a bass. It is very strange because the noise is independant of the volume and it happens even when master volume is off and no bass is plugged into the amp or a different cabinet is used. The sound does not change when I play with any of the knobs. But seems to occur when I knock lightly on the amp. Also the power light LED seems to dim slightly when the noise occurs. Any ideas what could be wrong? Is this repairable and worth repairing?