Screechy noise from engine?(with video) Please Help!!?

OK, so i have a 01 Mustang v6 3.8 and lately its been making an annoying screeching sound, stationary and while driving. I replaced the alternator with a brand new one and it still does it. And yhea i know the easy thing to do is to take it to a mechanic but i actually want to become a mechanic (i actually start college in September) so i want to fix it myself :)... but this problem has got me i have no idea. This is not my car but this is the EXACT sound it makes just like that a href="http://www.youtube.com/watch?v=Ya_rlzLZTOI" rel="nofollow"http://www.youtube.com/watch?v=Ya_rlzLZT…/a