Wierd/annoying idle noise

Ever since ive had my car...my engine makes an annoying woo-woo-wooo-woo(kinda hard to explain it) when its in idle..... and it goes away once i get my revs up past 1K..At first i thought it was the serp belt..soo i replaced it(needed to be replaced anyway) and it still makes the noise....Is this normal idle noise for a 97 V6 Mustang or is a worn out pulley or something??

it be nice if soemone answered me..its not that dumb of a question