Flutter Noise

[BatmanBob]

For about a year now I've been hearing this somewhat-whiney rapid flutter noise under the hood on my 07 (base model). It only happens while driving with the RPMs around 2500. It won't do it while in park which makes it harder to track down. There is no loss of power or jerking/stuttering. The tachometer needle doesn't waver either. The noise persists through shifts as long as I keep it around 2500 RPM. I tried to capture it on my phone, but it’s somewhat hard to hear: http://youtu.be/NITKXMLkcQc (it sounds louder than it does in the video)

Because it's just a noise, I've been lazy and haven't checked much. There aren't any blockages up to the intake and I've cleaned the air filter and the throttle body. Fuel injector cleaners haven't made a difference. The car has 80k miles on it.

There aren't any codes or anything, the noise is just getting quite annoying.

Thoughts?

[Slick2500]

Well that is weird.

[BatmanBob]

Anyone have any ideas on where I should start looking?

[NotEnoughGPs]

How many miles on the car? Have you ever had any under hood work or front end suspension work done to the car? Have you ever been in an accident?
Has the noise seemed to get louder over the past year? A year is a long time to be driving with a noise issue that does not go away. The good thing about modern autos is that there is mostly some indicator that a problem exists, and having noise is good thing as you can be made aware of a potential problem. (also, vibrations, odd smells, drivability issues)
I was able to access the Youtube video you created and listened carefully: It could be wheel bearings. Even if you have relatively low miles (for an '07 model, 70k to 90k miles is typical mileage) the wheel bearings (front or back) can start to wear and they make noise. It is common on these cars, even though yours is a '07 model.
Have you raised the car to look underneath to see if all appears normal?