I've got a 2001 206 diesel and, on a couple of occasions, it has started to make a loud noise while I'm driving. The noise sounds like an unsilenced engine (as though the exhaust had come loose or developed a hole) however after 20/30 minutes it stops. It has only done this a couple of times since I've had it (from new). There doesn't seem to be any loss of power when it happens, the engine doesn't heatup or seem to behave any different than normal (other than the noise) I assume it's a valve getting stuck open for a while, has anyone else had a similar experienceor know what the problem could be?