I got this horrible sound coming from the engine that sounds like a rattlesnake. It actually changes its pitch from high to low when I turn on the AC.

The car's performance is the same as before but this noise has me doing a lot of thinking since there's no codes.

Also, if I were to record the audio onto my iphone using it's mic app. How can I upload the m4a file onto this thing for you all to hear?

 

See if you can identify a little better the general area that it is coming from - sometimes that is hard to do I know. My first thought when I read your post was AC compressor because of the change in tone. Could also be a pulley bearing. We'll know more when you post a vid.

 

You are dead on

I took in to a shop and they said it was the AC compressor. Where should I go to buy one? The shop wants $755 for the whole job but they are charging 350 for the part.

 

You can get a rebuilt compressor at Autozone for $250. Check your local parts stores, make sure that it includes the pulley and clutch. Maybe you can get it cheaper on line, google it.

A repair shop will mark up the part to make a profit on it, standard business practice. But if you don't buy the part from them, they may not install it or will charge more. Ask about that.