2005 Xterra making a whining noise

My 05 Xterra started making a whining noise after I had the 30k service done. They replaced the idler pulley and fan belt (under warranty) Now it whines and it coming from under the hood. My warranty will run out soon and I need some advice as to what the problem is. You know how dealers are they don’t hear anything and send me on my way. What could it be?

as long as it is documented,and still no trouble found ,the dealer will still cover the part/parts under the warranty.as long as its noted.