First, try to locate the issue using the water spray method. Spray on the belt, if noise disappears temporarily or diminishes, it's an alignment issue, belt wear issue or belt contamination. If the noise gets worst, it's a tension issue. If noise does not change, it's likely a pulley issue.
In my case, noise did not change. I replaced all auxilary belts anyways. I used a gator back (4060615) and Maserati stretch belt for the water pump belt (214271), they looked like they needed replacing.
Second, I tried reconditioning the idler pulley (227894). They are sealed, so you need to pop the plastic, use carb cleaner, then grease it. In my case, nothing changed. I still replaced the pulley with a new one since there was some wear and tear on the bearing.
The issue I had was with the gasket on the crankshaft pulley. I've never had or heard of this issue, but that's what it was in my case. I took lithium grease, and greased the inner side of the pulley, as shown in the picture. Noise disappeared as I was greasing (I had the car on, and was greasing with a long needle). Noise is completely gone now.
Hope someone finds this helpful.