The weather is chilly. I've never had a FRST do this before - I know the normal noise that the car makes on start up in the cold for the first 30 secs or so but this is 10 times louder. Its like a hammer thumping the rocker - did it again just at lunch - seems to happen once the car is left sitting 2-3 hours.

Will try changing the oil but it was only done 1,500 miles ago.