Help! Humming Noise!

Hey guys I ended up selling the blue 1995 5.0 I had and bought a 1999 chrome yellow GT. It is damn near perfect, 61k miles, beautiful racing seats, I got it for only 6500. But... It's been about 3 weeks since I've boughten this and with the cold rainy weather in Michigan she's been in the garage lately. Yesterday I started it up and a LOUD humming noise was coming from under the hood. I took it for a drive and it stopped for a little bit. But it starts humming usually at about 1500 rpms. But if I tap the gas it stops. I've heard replacing the IAC valve tends to fix this. Any clues if that's what it is? PLEASE help! PS I uploaded some pics! Take a look :)
 

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