In my experience an idler pulley is usually a wine or whistle or is locked right up. AC clutch/ pulley makes a grinding or rattling noise.
You can check the idler pulley pretty easy, just take it off and stick it on your finger and spin it. It should spin freely and feel smooth. If its loose and catches or doesn't feel right then replace. The idler pulleys are cheap.
If it turns out to be an AC pulley issue that will be more in-depth.
RIP MY JEEPS... 1998-TJ 1992-XJ 1999-TJ 1994-XJ 1998-ZJ... All were lifted and locked...