Yep, my car now whistles. I got this car in October and the belts would squeal sometimes. They didn't look very new, and water on the belts stopped the squeal, so I changed the belts and that helped a lot, but then a week later my wife tells me about this noise. You know when you take a corrugated hose and swing it around your head, you can make a hollow-sounding whistle that goes up and down in harmonics the faster or slower you swing it? Yeah, that's it. The noise is not consistent, but generally starts when I hit the accelerator, the tone goes up when speeding up, but not every time and it also cuts out randomly. I came home and recorded under the hood, manually revving the engine. I couldn't pinpoint where the noise was coming from; it seemed to come from inside/underneath the car, but I'm not certain about that.
PLEASE tell me it isn't something like crankshaft bearings:

At first, I thought it was bearings going bad in the alternator causing the shaft to wobble like when you put a toy wheel on a shaft that's just a little too small and it wobbles when you spin it making a buzzing/whirring noise. So I swapped out the alternator with the one from our old car that was good and transferred over the almost-new mount bushings so I didn't have belt alignment issues. I gave the old alternator a spin when I got it out, and it definitely has bearing issues, but I still have the same whistling noise with the new one in. I also have a few recordings of it while driving, but the sound is quieter, being from inside the car instead of under the hood. Let me know if anybody would like to hear more examples

 

I seriously doubt it's a crankshaft bearing, no way. It's probably an accessory bearing but it still may be a belt related.
What happens when you spray water on the belts now? I have had strange noises coming from Volvo belts--once I had a water pump belt that made a rod knock sound, couldn't believe it! I don't think it's anything serious baring any other symptoms.

 

Spraying water on the belts now does nothing. While I had the belts off, I gave the AC and Power Steering pulleys a spin and tried jerking them around, and nothing was wobbling or making weird noises. Could it be the Timing Belt tensioner pulley bearings going bad? While trying to locate the noise moving my head around the engine compartment, it did seem to come from somewhere central, and the Timing Belt and accoutrements are right there front and center...

By the way, the car runs great otherwise, so I don't think there are any air leaks or other maladies.

 

It really could be any bearing. I have one of those stethoscope things with a probe at the end but it doesn't always help to pinpoint the source.

 

I had an opportunity today to open everything up and take a look. What I found was a severely-bound timing belt tensioner pulley. It would barely move, and the part of the plate that held the locator pin was bent a degree or two. Despite that, the timing belt looked to be in good condition, so I simply swapped the pulley from my parts car to the new one and buttoned it up. Quiet as a mouse. Well, a 4-cylinder internal combustion mouse...
Now I can better hear the valve ticking that I've suspected for the last two months...