Does it only knock with a load on the engine? or will it also do it if you build up the revs in park? If its only with a load, it could be detonation. If it still knocks with no load on the motor it could be something like a spun bearing. I had a 305 that spun a rod bearing once, and you could only hear it once the RPMs reached 3k. How does the oil look? is there any signs of metal in it?