CX500 - Source of Tapping Noise in Engine

[Barocca]

I'm hoping that one of our experienced members can help me to identify the source of a noise that is worrying me. 
The bike has been off the road for many months whilst triple bypass surgery was performed.  It runs standard exhaust system and air filter, has new timing chain, new piston rings (negligible wear on bores or pistons), Rae-san ignition, carbs rebuilt by CX guru.  The bike has been ridden about 40km and has had cylinder heads re-torqued and valve clearances set.  The noise is particularly noticeable at idle and low throttle openings.  There does not appear to be any leak around the exhaust system that might explain it.  Any ideas? 
Note that, given the other problems on the bike prior to the rebuild, it is possible that the noise was present before the bike was pulled apart.

[Seagrass]

Barocca that sounds like a very loud tappet noise.

How did you set the tappets (engine cold, drag fit with feeler gauge etc) and what gap did you set the tappets to? Did you turn the engien the correct way when setting the tappets?

Did you happen to start the engine before pouring 200 Ml's of oil down into the "camshaft well" (via the heads). If so this can cause excessive camshaft lobe wear so you may need to reset the tappet clearance again.

Seagrass

[CX PHREAK]

Tappets set on the right stroke?

 Did the gaps seem non existent when making the adjustment?

 Left you a post on the US forum.

[Barocca]

Seagrass & CXPhreak:  -  Tappets set on correct stroke(s), turning clockwise from front, at 3thou in, 4thou ex.  Cam lobes etc lubed before starting.

[Barocca]

I have just inspected the tappet clearances and found, in thou:
Left    -  Inlets 5 & 6  ,  Exhausts 5 & 6
Right  -  Inlets 5 & 5  ,  Exhausts 6 & 8

Have carefully reset these to 3thou and 4thou.  On start-up the same noise as before is present.  It appears to be coming from the right hand side of the engine. 

[fab]

maybe a bent or blocked pushrod? I would start it up and leave the tappet cover off and make sure you are getting oil up at the tappet and make sure everything is moving as its supposed too.

[doddsy1000]

It certainly seems like a tappet noise, doesn't it?

I've heard stories of worn rocker arms moving around fractionally while running.

Stories of a sticky valve also spring to mind.

Other than that, what everyone else has said. 

Especially running with the rocker cover off to isolate the location.

Wrap a big towel around the cylinder first to catch the oil.

[Cass]

I had the same problem so I threw the manual out the window and each time the valves were rocking I checked the gap took a few cycles however!

Cass

[Barocca]

Thanks All

UPDATE:
1978 CX500 with standard carbs, airbox and exhaust system;
Done triple bypass;
Rae-San Hall Effect ignition;
Carbs rebuilt by CXPhreak;
Big-ends plasti-gauged, minimal wear;
Piston/Bore wear minimal, new rings fitted, bores lightly honed;
Valve guides given only cursory inspection, valves lapped, new valve oil-seals fitted;
Rocker shafts are worn but have now been swapped (In<>Ex);
Oil circulation in cylinder head is good;
Compression test cold (psi): L-175 R-165, with oil down plugholes L-175 R-175;
Fan, housing and surrounds inspected.  All OK, no signs of any cracks, knocking or rubbing;
No exhaust leaks apparent near cylinder heads;
Mechanic's stethoscope confirms the noise is coming from upper right of engine (inside cylinder head?);

I got to thinking back to 2015 when I first took the bike off the road.  My post https://www.australiancx.asn.au/forum/index.php?topic=13721.msg141483#msg141483 includes the following: "The last time I rode the CX500 it started to make a clicking sound as we neared home.  It was sufficiently different from normal for me to consider whether to stop immediately or continue the remaining km to home.  There was no misfiring or loss of power, so I nursed the bike home.  I’d only done the basics after buying the bike (comp. test, tappets, oil and filters, plugs, flush and change coolant)." 
I'm increasingly wondering whether my current engine noise is the same one that alarmed me in 2015, it certainly sounds very similar.  If it has the same origin, then it may provide some pointers.  For instance, the noise seemed to begin suddenly near the end of a 100km ride, and, the work that I have done since has neither unveiled the source of the noise nor remedied it.  Curiouser and curiouser!