[Meotchh]

Hmm sounds like it could be a bad amplifier, a loose ground or a speaker wire shorting to ground? Those are the three areas I would concern myself with. Not knowing much about the reliability of the Mach460 system I would first check the grounds before opting for a new amp.
It is not "interference" from the altenator. Rather it is the induced noise of the charging system. This could also be caused by loose component(s) inside the amplifier.