Dirt in the volume and/or balance controls, time worn solder joints that are loose, failing capacitors, loose connections, etc. You name it. Older equipment may have these issues. It has nothing at all as to whether it is Mac, or any other brand. Sometimes you can spray electrical cleaner in the volume/balance pots, (with the unit unplugged) and turn the controls many times back and forth while spraying and this may clear up the noise issue. You can tell if this is the problem by turning the volume/balance control and if the noise follows the control adjustment, that is typically it. If not, sometimes a complete resolder of all internal circuit board connections is required, as cold solder joint has presented itself. Nice vintage equipment is definitely worth this effort. Some of the older Mac, JVC, Sansui, Pioneer, etc. equipment many with the solid wood cabinets are worth it definitely.
enjoy
