Had another listen today. No knock for about 30 seconds then it started up. Loudest on the side of the block at number 4 cylinder and also on the exhaust manifold in the same position. Distributor sounds fine but the back end of the head is also noisy. Sounds like a gentle hammer tap. I had a local mechanic take a look and he thought maybe small end bearing. If it is that is it OK to drive without doing more damage?