100f high pitch noise from powerhead?

[kennethgi]

hi, my name is kenneth, im new to the forum. i have a yamaha 100f. my problem is that there is now a high pitch or high frequency sound emanating from the powerhead occasionally. i've identified the lower front part, below the electronics. this noise is most obvious after decreasing throttle to idling speed. mechanically the engine sounds solid, no ratling, oil level normal, the engine temperature is normal. thinking now, battery is at least 6-7 years from previous engine, but engine starts great. i run in salt water but not have facility to flush engine after use with fresh water. the engine is due for service after the next 10 hrs. from my professional experience in electrical engineering it sounds like a frequency inverter con*****ing an electrical motor speed/frequency.
my question is, could a bad battery cause the rectifying unit to overload at mid range throttle, overheat and cause this high pitch noise which gragually degrades after a few minutes when throttle is lowered to idling speed, any suggestions appreciated.
i will replace the battery with a new one and post results
thanks kenneth