When come to a stop in my E320CDI there is a 'whirring noise' from the engine which lasts approx 5-10 seconds. I also think I've detected it when in motion. Could anyone please advise me what it is? presumably it's an engine function which is meant to happen? Never heard it on my E240 though. Someone told me it was the diesel engine cleaning it's own filter by injecting fuel into it but I'm not sure what they meant by this.