Very odd but my Markbass F1 head on a couple of recent occasions has made shorting type sounds thru the speakers of the connected cabinet and the speakers jerk in and out violently. It does this even with no instrument cable plugged in. I've been able to determine thru the process of elimination that its not the speaker cable or the cabinet so it must be originating in the head. I don't even know what to call this to send it to a repair center. Anybody got a clue what this might be.