Yup sounds like a lifter to me too.
It could be a bad rocker arm.
There are tin "caps" in the cups of the rockers and they break loose and rattle around between the rocker arm and the lifter.
it can do that forever and do no harm.
I would not go into the engine just for that either.
Now if you were doing head gaskets then yes, I would replace all lifters and rocker arms.

 

there are 4 things you can do to check this.
Get stethoscope or piece of tubing about 3 feet long and hold it near any noise it should assist in pinpointing the location.

Next determine if there is an actual clearance problem in your engine.

This is an easy step
With the engine hot, Get a black piece of cardboard and take out the dip stick.
With a piece of lamp cord , (tie a knot on one end so it wont fall into the tube) put it into the dipstick hole till the cord goes into the engine oil, you might be able to only use one strand of the cord so peel it apart.
Note: I have found putting the dipstick into the tube makes little pieces of metal so thats why to use the lamp cord its plastic so it wont damage the dipstick tube and give a false reading

Dip the dripping oil onto the cardboard and after a few dips you should have a little puddle of oil.
Go into the sunshine ( a magnifying glass is good here) then inspect the oil for metallic particles.
They will show up very well with the black back round.
If you find any then its good chance your motor is making metal.
Change the oil and recheck after 500 miles. Depending on the amount of metallic particles you find

With motor still warm remove the spark plugs and do a compression check.
This will let you know if you have a burned valve or possibly a collapsed lifter if you find a cylinder with low numbers, or other valve train problems like a noisy rocker arm.

Next go to NAPA and buy a block checker 50.00 .
This instrument is for checking for hydrocarbons in the coolant, it draws off the vapors through a tube filled with blue fluid while the engine is running,
if hydrocarbons are present the fluid will turn yellow, then it indicates a head gasket, or head crack ,or liner failure. Follow the instructions in the kit

From the info provided you may have a loose rocker arm or a worn pushrod or a collapsed lifter, you might try adding a quart of Marvel Mystery oil to the oil pan and driving the truck for 1000 miles (this is mindful of the results of your previous tests).
Following this info should let you zero in on your mysterious noise. ( the MMOil will assist in cleaning out deposits in the lifters)

Dont forget to put a wrench on all of your exhaust manifold bolts to block and the header pipes to manifold, you will be amazed at how they can be loose and look tight

 

you first need to determine the cause of the noise, or at least eliminate some causes. These Rover engines of ours are seemingly prone to top end noise, It may be a simple matter of rebuilding the rocker shaft assys and replacing the pushrods.........a fairly easy job. There is often only a few thousands of an inch wear between noisy and quiet.
The valve rockers have oil passage holes in them that can become plugged for a short time and cause a noisey valve train. My previous owner(s) weren't really into vehicle upkeep and I dealt with this on a first hand basis. It might be worth looking into.

 

http://www.hicountryoffroad.com/Prod..._skidplate.htm

 

Where did the skid plate link come from?! Or did i miss a big change of topic...