Humming, possible alternator/belt problem?

[0411gogo]

For the past month, my 2000 X has been making a loud humming in the engine sporadically. At first, it was only when the AC was on. After a 1000 mile trip from Portland to Nevada (which went perfectly smooth, no humming at all), it's been humming more frequently and without the AC on. I was sitting at a light when it was humming, and I turned the AC on to test it, and it completely killed it. It restarted just fine.
Over the phone, a mechanic told me it was probably the belt or alternator. Any guesses or advice on what to look for first?
Also, how much should an alternator cost to replace, if I have it done by a friend instead of a shop?

Thanks in advance, any input is helpful!

 

[0411gogo]

Reading more, I think it might be the IAC. If I can just clean that, hopefully the problem will end. If not, any guesses?