Unusual Sound 308 Engine?

Hoping for some insight ........

I have an unusual intermittant issue from my 308 engine.

At no particular frequency (other than occassionally) when my engine is up to temperature and sitting at idle revs for a prolonged period I get what I can best describe as a "clack" sound appearing to be coming from the engine on the left hand side (looking from the rear)

The engine briefly stutters/ shudders slightly (as in less than a second) and continues as normal. The "clack" does coincide with the engine, "jumping" slightly - i.e you can feel it if sitting in the car.

It may not occur again while the car sits and idles, or it may happen again in a couple of minutes.

My engine as been rebuilt recently, a full rebuild, it has less than 500 miles on it since rebuild. It has 10.5:1 JE pistons. Other than that it is a stock engine.

Carbs are properly balanced and engine is tuned for HC and CO.

Since I first noticed this I have have new ignition leads, new plug extenders, new plugs, but it still appears to be there. Both coils are working properly.

There appears to be no issue on driving, car runs fine, pulls fine etc...... but this is annoying me, and it quite noticable, almost like metal whacking metal.

I am based in Ireland, fuel is 95RON....... wondering if I am getting occassional detonation with the higher compression....?

Has anyone out there come across this before/ any suggestions what might cause?

Also posting in Tech forum.

Thx in advance