Even on auto engines, knock can occur long before it is audible to most people and it is mostly a problem at high loads where the engine is generating a lot of noise anyway.
An exhaust leak on the other hand, even a tiny one, can be pretty obvious especially at idle. This was (presumably) a gas leak with at least an order of magnitude increase in pressure behind it. Being only on one cylinder will make it a distinct ticking/hissing sound, I would have thought.
Last edited by Andy_RR; 11th Mar 2016 at 05:10.
Reason: Grandma