Ghosts in the Machine: Used Sailboat Adventures

My otherwise faithful Balmar Alternator and external regulator recently sound as if they’re revving-up and then down.   Two to four seconds pass, then it reoccurs. The sound corresponds with: 

• ·      The battery voltage moving up and down
• ·      A green light flash on the regulator (May not be 100% correlation).
• ·      A Tachometer that waves at me.

These used sailboat symptoms appear less noticeable while under load and after the engine runs for a few hours.  Both devices are quite old but I don’t have dates.  They didn’t always make this sound.   The alternator belt is in good condition and doesn’t appear to slip.  

Questions: 

• What would cause this?
• Are my Batteries at risk?
• The alternator is charging a bus which is connected to three different banks is that an issue?

I look forward to posting my solution soon.

Measurement Details and Pictures Below:

Engine Run time (.3=30 sec)	Battery	Measurement tool	Low Voltage	High Voltage	Delta in volts
Off	Starter	Volt Meter	12.4	NA	NA
Off	Starter	Gauge	12.7	NA	NA
Off	House	Volt Meter	12.9	NA	NA
Off	House	Energy Monitor	12.96	NA	NA
Off	House	Gauge	13.2	NA	NA
Off	Regulator- Blk&Red	Volt Meter	12.9	NA	NA
0.3	Starter	Gauge	13.5	15	1.5
0.5	House	Gauge	14.5	15	0.5
2.4	Starter	Volt Meter	13.2	13.8	0.6
3.5	House	Volt Meter	13.9	14.3	0.4
4.57	Regulator- Blk&Red	Volt Meter	13.8	14.2	0.4
14.36	House	Gauge	13.6	15	1.4
15	House	Energy Monitor	14.1	14.5	0.4
17	Starter	Volt Meter	13.1	14.6	1.5
18	House	Volt Meter	14.1	14.3	0.2