3.8 V6 Windstar engine has a knock/rattle.

I used a long screwdriver against my ear, not ideal I know, but everything was pretty quiet except the belt tensioner. It's relatively inexpensive, so I think I'll just start there. If that doesn't do the trick, I guess I'll get a stethoscope and try from there. Thanks for all your help.

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Update, ordered a belt tensioner from rockauto.com for under $20 including shipping, came in today, slapped it on, all is quiet again. Never heard a tensioner make a rattling sound, sounded kinda like a catalytic converter heat sheild rattling, but oh well, all is good. Thanks for the suggestions for troubleshooting it.

You bet, glad you got it fixed! .. Thanks for the feedback! .. May help others with same problem...

I know it has been a while, but my noise was gone for about a week after the new belt tensioner, then came back full force. Was at my wits end with it, then I was on another forum (for the Aztek of all things) and some people had the same issue, and come to find out the s-belt they replaced was just a little longer than the original. I still had the original, and checked the length of the newer one, and sure enough, the newer one was about 3/4 in longer. Put the old one back on, and heard the sweet sound of no rattles, just smooth engine. The newer belt was fine for about 2 1/2 months before the noise started, so it never occurred to me that that could be the issue. Hope this helps someone in the future, be sure to check belt length.

In addition - when purchasing parts, be sure to find an experienced parts counter sales person! .. Application listing clearly shows belt length deferences.... There are 3 possiblilties for this vehicle (Napaonline.com) .. Just saying.... Best to take the old part with you to the parts store for physical//numerical comparison, in most cases.