I have a very stock 95 3.0 with 87K and recently noticed a high frequency whine from the speakers that changes in pitch with RPM. I hear it through the speakers with or without the radio turned on (?). The radio itself is the factory cassette deck and everything electrical is stock as is the rest of the 4Runner.

I suspect it is either a bad ground somewhere or a failing alternator although the voltage gage in the dash is reading fine.

Wires and plugs are due for a change soon due to mileage but nothing about them indicates an outright failure (no bad misses, hard starts or arching). Cap and rotor was changed last week. Battery is about 2 years old and *should* still be fine. Battery terminals are clean and clamps tight.

What is the most likely cause of this?

 

I agree about the bad ground assumtion. I would imagine there is a ground strap behind the stereo that needs to be cleaned. I would start there. I would take my 4runner down to a shop that can do a load test on the alt. to rule that out. And clean the battery terminals just incase. If not then my assumptions are worthless. Good luck, Blue