I am convinced it's a temperature thing.
This weekend (warmer weather), I have tried the car from cold and experienced much less noise from the engine.
On Sunday, I didn't go out in the car till the afternoon and it was very warm outside. The car hardly made any noise and after a couple of minutes, it was gone completely.
Coincidence??? I don't think so. Not that it makes me any happier, because it may mean there is no fix.
