My '98 Ford Windstar (3.8L) has a really loud "unusual" idle noise. It's not a knocking, just a constant loud noise. It usually persists until you get the van in a high gear at a good speed, and even then its still loud.
I believe the problem is related to the idler pulley? and serpantine belt (see the pics). The backmost pulley in the picture is REALLY far off center, and vibrates a lot when the motor is running. This is a cause for concern, is it not?
There are some bolts right around that area that look like they would fix the issue (by forcing the pulley back in line with the others), but the bolts in question are snug tight. Sorry if the image isn't good... I can take more at different angles if needed.
Any help is greatly appreciated!