Noises problem
2007 Ford Focus 4 cyl Automatic 46000 miles
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I have noticed when I drive my car for more than 20 minutes or so I constantly hear a loud whirling sound, like a loose belt or something in the engine area. Its not a tire, or the brakes, we just had the rotars, and break pads changed. It is a constant noise, no matter how fast or slow I drive. The noise has slowly been getting louder, now it even sounds a bit squeeky.

I was looking in the engine, to see if i could locate the noise, and I couldn't. I cant even find the belt, so I cant check that.. What do you think this srange noise could be??