Here is an interesting thread:

http://www.peachparts.com/shopforum/showthread.php?t=189663&highlight=ovp+buzzing

I searched on "OVP" and "buzzing"

It is a different car but similar symptoms. You might verify the voltage when the car is running just to make sure the voltage regulator is OK. At some point you may have to just try to find a decent mechanic. (hard to do). The ones that understand electrical systems are few and far between unfortunately it seems.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra