Although Citroen seemed to have other ideas about which oil grades were suitable for their engines...

Dyane Owner's Manual 1979 by slcchassis, on Flickr

Although Citroen seemed to have other ideas about which oil grades were suitable for their engines...

Dyane Owner's Manual 1979 by slcchassis, on Flickr

Regarding tappet clearances, with unleaded fuel and unless you want to check them at much shorter intervals than the 10,000 miles suggested in Citroen's "Maintenance Guide" provided with every new vehicle, it's best to set them slightly wider than originally recommended.
Using a 0.25mm 'slack fit' for the exhausts and the same but with a 'sliding fit' for the inlets works well.

Ken, watch out with these pics.... Interpol has your
fingerprints now...

And DNA

Sounds similar to how my dad's D*lly did after it suffered a partial seizure in 1995.

Silly question, It isn't the rocker tapping the rocker cover ??

It does sound like it doesn't it? But no it's not tappet or rocker related. It'll be piston/Little end/slap/partial seizure related.

Bonnet slid off last night and I'm all geared up for the engine out and strip down.

Thanks for the tips on gaps Ken (you built my Chassis and rather a splendid one it is too), I used 0.20mm and 0.15mm years ago but did have a valve close up on me once, so 0.25mm for both and slack/sliding is a sound suggestion, I know the racer boys use even wider gaps than those at 0.30mm+

Have you changed the spark plugs?
I had a noise like that once, turned out I had stupidly used a set of two wrong plugs which were too long and one was contacting the piston at TDC.
Its probably not this, but easy to check!

It looks like we've got to the bottom of it..

Partial seizure. New Barrels and Pistons are on their way.

What causes this?

Lack of lubrication ?
check if there is oil comming through the oiltube on the barrel .