Hi , I've put a new cylinder head and gasket on my Defender , initially had problems with getting the injectors properly clamped down , sprays of diesel shooting up from the head , took several starts before I managed to sort them all , that's done now and it starts and runs well but I haven't let it run for any more than a minute as there is a loud popping/thumping noise coming from the top of the snorkel ! Can anyone tell me what on earth it is ?