I believe the SPA 400 does not use shielded cables. If the problem's just started, have a look at bonds and shields on all wiring. If OK, I'd go with the genny or voltage regulator (is it carbon pile or vibrating contact? (old!))
With genny online:
Switch off comm - can you still hear interference through intercom?
Switch off intercom - can you still hear interference with comm quiescent?