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...
