FORD WINDSTAR ENGINE KNOCK

I Have A 2000 Ford Windstar, With A 3.8 Liter Engine With 159,000 Miles. The Engine Starts Up First Time And Sounds Great During Start Up...

What Causes Knocking In The Engine And What Needed To Repair It

I Just Bought This Minivan2 Days Ago, It Is In Great Shape Like It Was Well Taken Care Of But Now I Have Noticed Apretty Distinct Knocking...