I have a 2011 Vauxhall Meriva 1.3 diesel. When driving I get a serious rattling sound from the engine. It is a tinney sound like loose tappets or timing chain. It doesn't do it all the time but usually happens when I am driving steadily, at a constant speed, between 30 and 50.

When the rattling starts, if I accelerate slowly the noise gets louder and if I accelerate hard it disappears after a couple of seconds.

I have been told to check my oil, which I have done, because, when the DPF (diesel particle filter) gets blocked or the soot in the filter reaches a set limit (about 45 per cent) the vehicle's ECU will initiate post combustion fuel injection to increase the exhaust temperature and trigger regeneration.

If the regeneration is unsuccessful due to an insufficient driving cycle the extra fuel injected into the cylinders will not burn and will drain into the sump. As a result, oil quality will deteriorate and the level will rise. It is important that you check that the oil level does not increase above the maximum level on the dipstick as diesel engines can run on their own oil if the level is excessive – often to the point of destruction.

My oil level was higher than the max so I booked it in for a service, oil change etc and for the rattle to be checked out The car is still the same and the garage cant find anything wrong. Any ideas?