I have a distinct ‘tapping’ from the engine and cant easily identify the source.
I am thinking the cam area and possibly the tappet buckets?
It may be something else (hopefully easier to fix)
I removed the filler cap while running to see if it is noticeable but I cant tell.
Im sure there will be those familiar with the engine who know what the distinct sound is.
I have a video of it running…
That’s a heavy tapping if you use a long screw driver and hold it against the cam covers you should be able to isolate the area more specifically.
Something is seriously amiss Philip… and it must be from the valve train as it coincides with the cams rotation.
Take the valve cover off and measure the valve clearances, it might give you a hint.
Tappet guide! Take the valve covers off. Do not run the engine. Start with the exhaust valve cover. Sounds a lot like mine when a guide came up.
What’s the whirring noise?
That’s serious: remove the cam covers and start inspecting tappet guides.
If one has risen, it can be tapped down and affixed by a stakedown kit. I’ll guess exhaust side.
Is it as pronounced in person as it sounds like in the video.
That’s what my faulty tappet guide sounded like, though mine wasn’t as loud? I sent the head off to be rebuilt.
Thats the exact same sound I heard on my 57’ Mk1 when the tappet guide became loose. You could actually see the guilty tappet by looking through the oil filler. I would go ahead and pull the cam cover, bet it will be obvious. Jeff S.
IF it IS the cam striking the tappet guide, you will see marks on the top edge of the guide…inspect…fix as Paul said…and also…clean out completely the oil valley in the cam areas…(both sides), flush some oil with a little pump thru the cam oil way if you can, change oil and filter…as little bits of metal likely will have been chipped off the tappet guide edge. Nick
Why?
The oil in the other cam area will be the only part of the engine that has no pieces of tappet guide. Remove cover, find issue, I say it’s likely a guide; knock it down after pulling the crankshaft. Fit a stake down kit. Clean the debris (if there is any) from the area and put it all together. Now run it briefly and then change the oil.
If something goes wrong it will be in the oil pump and that‘s not easy to avoid. Contaminants collect in the puddle and may go down to the gallery, into the sump or into the timing case go go into the sump. If caught early there will only be a couple dents and no further trouble.
…Then realize that you should have removed the camshaft. Put the crankshaft back with an embarassed look over your shoulder, checking that nobody noticed your error. Now remove the camshaft before knocking down the tappet guide… 
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Oops… 
Sounds like something I would do though.
Shut it down
Good luck
I have used an engineers stethoscope, but still find it hard to identify the area. However, it is a lot noisier in the exhaust side cam cover than the inlet.
I did rebuild the head and set the valve clearances… and was very happy with the measurements for clearance.
Will it be obvious to look at? what are the tell tales?
I bought a stakedown kit in anticipation. If one has risen…will it stay risen and be obvious which is the culprit?
What is the likely outcome if it is run at low revs as it is (to move it in and out of the garage)?
Yes it is…sometimes quieter than others…but still obvious
Is there a knack to prevent the oil sat around the guides (low side) from spilling down the head when the cam cover is removed?
The dents will be in the follower I presume?
Does the camshaft have to be removed to knock the follower down? If it is a follower thats causing the tapping…will it definitely be sat high of could it be sat back down?