suspected turbo failure on this owned from new 7500 mile 2013 DS3.
took it to the Citroen dealer we bought it from and they have confirmed the turbo has failed. prolonged use would have lead to diesel over run and a complete new engine.
thankfully we spotted this before anything bad happened saving the engine
the car is still under warranty so we got this fixed for free.. however the original engine oil was reused (which is a must change thing when you have a new turbo, we complained and it plus a filter was changed for new) and within a week we started having EGR faults. however that was down to an oil coated EGR valve as it was not cleaned. all in total it had to go in for repairs 3 times.