I own a 2006 Impreza sedan with 134k miles. When I started the car today I heard a loud squeal. I couldn't tell if it was belt, idler pulley or AC clutch bearing. I removed the idler pulley and checked it. It seems to be rolling fine and not binding. Once I put it back together the squeal was gone until I turned on the fan. When the fan was on it would squeal, when off no squeal. Anyone else experience this? Thoughts on repair. Thanks

Is the AC or front defrost on? Either of these settings will engage the AC compressor when the fan is on.

 

Turn the fan on the vent or floor setting and turn the AC off and on. Does the squeal happen only with the AC on? If so, the AC belt is loose and probably needs to be replaced.

The fans are actually a pretty good electrical load. Sounds like you have a tired Alt belt. If you can make the squeel on demand, then just get some water on the belt, if the squeel changes away it is the belt. 

Check pulleys for grinding noise also, you can use an stethoscope to hear into the pulley; please don't do this with the engine running