So I replaced the idle control valve. It was about $50 and very easy to install on my '91 Cherokee. It dang near redlined for 2-3 minutes after I started it up because the valve was open and hadn't adjusted to the right idle RPM yet, but it flattened out.
However, no luck. The whine crept up...
I have the same situation with my 1991 Cherokee (6 cylinder). Of course it's fuel injected, so it utilizes a Manifold Absolute Pressure (MAP) sensor to regulate fuel/air density.
Same things happens to me though....I accelerate and the whine cuts off, but immediately after letting off the gas...