2002 Ford Explorer whining

Noise from the generator or from the radio? How old is the battery?

A battery more than two years old can cause problems with GM's disastrous generator design. Ford voltage regulators work the same way, and while they can cause the same problems, I haven't heard of that as much. The heart of the generator is a coil of wire, just like in an ignition coil, and the voltage regulator switches it on and off, ... Just like an ignition coil. As a result, they develop a lot of voltage spikes. The battery is the key component in absorbing and damping those spikes, but as they age and the lead flakes off the plates, they lose their ability to do that. On GM vehicles that results in numerous repeat generator failures. On other brands it usually results in a whining noise from the radio.

Since you're hearing this under the hood, the best approach is to use a stethoscope and listen next to the generator pulley, then next to the tensioner and idler pulleys. The source should be pretty obvious when you listen right next to it that way.

Ford had six or seven service bulletins for various attempts at solving a belt squeal, none of which were very effective, but that's a lot different sound than a whining noise. You'll need to have someone listen to it to tell what is causing it. A belt squeal will change when you dribble a little water on the smooth back side of the belt while the engine is running. The noise may get louder or quieter, but the point is it will change. If it does, one of the pulleys is turned or tipped a little. That makes the belt slide across that pulley or the one right after it as it goes around it.