Wayne I followed your lead per the PM and found this on the dieselpage
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The lifter takes up the slack in the pushrod to rocker clearance using engine oil as a hydraulic fluid. A worn or damaged lifter collapses and eventually won't lift the pushrod enough to actually work the valve - there will be a misfire as well as noise associated with this.
First make sure the knock/clack is at half engine speed.
If it is, then use a screwdriver to pinpoint which head/cylinder it's coming from. If you're going to replace all of the lifters at the same time, you don't need to worry too much.
It's possible to remove the lifters without taking the head off. You'll need a great deal of patience, a mirror, magnetic pickups, long screwdriver etc.
With the rocker covers, rocker assemblies and pushrods removed, unbolt the lifter retainer plates and use the mag pickup to pull them out of the tappet chests.
Now use the mag pickup through the pushrod holes and main tappet chest slots to pull the lifters out.
Replacement is more tricky using the mag pick up/screwdriver combo, since not only will you have to maneuver the lifter into the lifter hole, but you will also need to align them so that the retainer plate fits properly. It's possible to bolt the retainer plate over misaligned lifters and destroy them within a few miles.
It's definitely doable without taking the heads off and I've done it twice now using this method - just don't rush it
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Thanks