I can't make the sound change by adjusting the exhaust nuts. Youtube does degrade the already poor quality sound, but it is a tick rather than a hiss.
Damned if I can locate it precisely by listening through spoon handle etc - I certainly get a definite clacking coming from the front rocker cover, but that may be just the normal clatter. It's definitely louder than the rear cover. These sounds overwhelm the much higher frequency ticking sound when listening through a handle or tube. But I would say it's coming from nearer the top than the bottom.
I ran the engine in the garage for fifteen minutes, but it wasn't as hot as yesterday. The sound appears after a few minutes, and occasionally disappears for a few beats. It increases as the engine gets hot on a run.
Cold and raining here, and getting dark. Back on the job tomorrow.....