Ford Repair: Ford Explorer - Camshaft Chain Tensioner, chain tensioners, cam chain

Question
My Explorer (1998 / 4.0L 69,500 miles) is making a noise noticable at idle speed.  Makes the engine sound a bit more like a diesel than the gas engine that it is.
The dealer says the camshaft tensioner needs adjustment.  This maintenance does not appear in my maintenance schedule and I am wondering why this is an issue for me.  The service charge is quite high (>$500) and I wanted to know if it is really necessary.
Thank you.

Answer
Chris:

The repair suggested is probably replacement of one of the four cam chain tensioners on the 4.0 sohc.  There is no scheduled maintenance due to this portion of the vehicle.  If the tensioner fails completely it will allow the cam chain to slip which may cause the valve to hit the piston, which will at least require a cylinder head and piston if not a complete engine replacement.

Patrick