The fan should be stiff to spin when the engine is hot. If it free wheels, there is a very good chance that the fan clutch has had it. My 260e has a whistling sound coming from the alternator area. The alternator has a lot of free movement on the pulley indicating the bearing is on the way out. If the tensioner is not installed exactly correctly you can get a squeal squeak type of noise that seems to come from the fan belt.