B. The Hydraulic Tensioner Should be VERY Stiff to operate by Hand.
[When removed]Otherwise it's in need of replacement.
[A New(Or DRY) Tensioner Should be Immersed in Clean container of FRESH
Motor Oil and whilst immersed "Operated" to fill it with oil.]

If you can get Whatever "IT" is to CLACK whilst stationary:
1.Motor or Transmission mounts(Specific Height Tests for the Motor Mounts)
[Tight Bolts, Means Bupkis]
2.Timing Chain Apparatus
3.The aforementioned (And Accursed) Serpentine Belt Tension-ing System.

Do Not Discount the Injectors!

Harbor Freight:Inexpensive Mechanic's Stethoscope...Invaluable!

"Fan clutch – 25K miles on it and uggh, has bearing play, but not source of noise I think."
(Suspicious!)

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'84 300SD sold
124.128