Ok that makes sense since the GPs' heat plus the compression heat making it silent. After all higher cetane in fuel makes it ignite at once and makes the motor much quieter, so this in a way will also make it combust at once because it is so much hotter than normal. Come to think of it, normally the longest it is quiet is after 5 cylinders fire.
My GP relay is probably original so I don't think it stays on after the car starts (it is only the new ones which do this right?), however I'd imagine there is a certain amount of heat they are still letting off even after shut off.
Thanks for the input everyone.