Fabia Estate (2010) Mk 2, 1.2 TSI DSG: - Can anyone tell me what this rattle is and how to fix it?

Background: -

1. May 21 st 2014 - DSG gearbox oil was changed, at 15940 miles, from synthetic to mineral oil free of charge under Skoda campaign reference 34F7.

2. Feb 19th 2015 - Minor front end impact damage repaired, at 23000 miles, by insurance company specified repairer.

3. The DSG has performed smoothly to date so I do not think the noise is DSG related.

Noise: -

4. Mid-March 2015 – At 23500 miles an annoying random rattling noise when accelerating began to happen. This 150 “taps per minute” rattle is a ‘solid’ rattle like a hammer hitting solid metal (but muted by the engine bay sound insulation) so I do not think it is pinking which is a more high pitched tinkling sound. My initial thought was that a mechanic (from the February repair) had left a spanner in the engine compartment which was rattling against something when accelerating but mechanics say it is not that.

5. The rattle is random and, when it does happen, it only happens under acceleration. It seems to come from the engine area and it can start and stop suddenly. I.e. I could have driven 10 or 50 miles with no noise then at the next acceleration it can suddenly happen for the next few miles then stop again.

6. Since June 2015 – Started to use premium grade fuel plus RedeX fuel additive to clean the combustion areas. The noise, when accelerating, still happens sometimes but it is not as loud or as frequent as it was. It may not happen for three weeks then make reappearance.

7. Could it be related to the direct injection engine design and the use of cheap fuel causing carbon build-up on the inlet valves which then breaks off in pieces and rattle around the combustion area before passing out through the exhaust valve and entering the exhaust/catalytic converter area? If so then the use of premium grade fuel and RedeX fuel additive has not removed the carbon. It would explain the random nature of the noise but not why it only happens when accelerating and would it be a ‘solid’ rattling noise as described above?

8. The random nature means that two separate garages heard the noise whilst driving the car but it failed to repeat itself in the workshop (not accelerating) so they have been unable to identify it. Both garages confirm that the exhaust and heat shield are not loose. Internet and Briskoda Forum searches have revealed nothing.

9. March 2016 - The car passed its MOT (emissions) test carried out by main Skoda dealer so exhaust and catalytic converter must be good.