Imagine this: I'm going down hill at about 60mph, in fourth. The engine is working away quite happily (compression is good on both sides; engine happily does 75mph on the flat, maybe more if could be bothered to push it, what with the wind noise and everything).
However, when I take my foot off the gas the engine can be heard giving a fairly faint, but nevertheless very audible, metallic popping noise, randomly yet frequently.

Same happens if I'm going down hill in third, at say 20-30mph.

Any ideas what this is... and should it be cause for concern?

2CV engines are fitted with carburettors, so there's always a small amount of mixture which will be drawn into the engine when it's on the overrun, even with the throttle closed.

Every now and again, the mixture in the combustion chamber will be _just_ rich enough to be ignited, but as the mixture is likely to be on the lean side, its rate of combustion will be slow and may not be complete when the exhaust valve opens.

So, it's still burning as it enters the exhaust system and can ignite the unburnt mixture which is already there, producing the noises you're hearing ...

ken

tl:dr It's characteristic...

Aaaahhh!

I can honestly say that with my previous 2CVs I never noticed that sound! Mind you, they did have rather loud music systems in them, whereas my present snail has no radio at all, forcing me to pay more attention to other sounds and sights.

Thanks for that, Ken.

BTW, your post wasn't tl:dr. It was just what I needed.

i get a tapping popping noise (depends how you describe it really) on idle

tappets?

i thought it could be, havent checked them since may, will check them soon

Yep all the A series i've had or driven do the popping down hill with throttle off.
But whats a tl:dr

tl:dr = "too long: didn't read"... used as a putdown reply to an overly long written statement.

Seems a tad rude to me, but I mustn't go on!

http://en.wikipedia.org/wiki/Wikipedia: ... idn't_read

Maybe Ken was just giving the highlights of his post rather than commenting on your post?

Yes, that's the way I understood it!
Sorry for any confusion: my comment was on the derivation of "tl:dr", not any perceived subtext to the use of the term in Ken's reply.