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With the engine not running the voltage would be even lower than at idle, moving the evap RPM lower than the "sweet spot" that causes the harmonics, when you rev the engine up it moves the evap RPM above the "sweet spot" Does the evap have more than one speed???

You are lucky enough to hit that "sweet spot" under normal voltage conditions, you could try a resistor to slow the fan down just a little bit.

Normally the 200A uses three belts, I have a modified 200A (conversion) mount,, IIRC the mount moves the alt back just enough to use three belts on the alt and only one on the PS pump (the three alt belts also go around the PS pump) that leaves the fifth groove open on the crank pulley for the...

SORRY!! I was referring to the OP, he was the one that said it only happened with the AC on and at idle, I will be willing to bet if he puts a voltmeter on the truck and watches the sound (caused by the harmonics of the fan blades turning at a particular speed) it is directly affected by the...

OK, seeing as you have a 60A alternator it just might be the voltage is sagging at idle and causing the evap motor to get to a speed that causes it to make that noise, that would explain why it only happens at or near idle, maybe time for an alternator upgrade!

If it is one would think it would change whilst turning the steering wheel back and forth, something to try, although the AC having to be on seems to rule out everything but an AC issue, but.....