Just wondering if your 'Boing!' sound may have happened again yet and if you have discovered the cause?
I'm asking because my Pic has started making what appears to be the same noise, a single loud electronic 'Boing!' from inside the dash on starting.
Mine's a 06 plate 1.6HDi, and unlike yours it's making this noise almost every time I start from cold. Once it's warmed up it seems to be more random, maybe 50% of the time.
This is obviously a noise that Citroen have 'built-in' as a warning for something, but I can't find any reference to what it is in the handbook or online. There's definitely no warning lights or messages on the dash at the same time. Battery seems very healthy.
(One possibly unrelated thing: over the last couple of months my engine warning light has started to flick on occasionally while driving - high pitched beeps, engine looses power, take my foot off the throttle a little, then power comes back, light off, everything back to normal. Was hoping to get by 'till next service before having to figure this one out!).
Anyway, sorry, didn't want to hijack your thread rawthey, but as the Boing noise on starting seemed so similar thought it may be a good idea to keep all queries about it in the same place.
Anyone got any further thoughts about what it might be?