Knocks that stop when the ignition is disabled are most often caused by a cracked or broken piston skirt. Very rarely, you'll find the wrist pin has worn excessive clearance into the piston. Since the rod clamps onto the wrist pin on a stovebolt, a worn upper rod bushing is not a possibility on these engines. A connecting rod knock will change pitch, or sometimes become a double-knock noise, but they seldom disappear entirely. Bottom line, you need to pull the cylinder head and oil pan, and find our what's going on. Once it's that far apart, you can decide whether to do a patch-up job or go ahead with a full rebuild.
Jerry
