FWIW, I had a previous TR3A engine that made a similar knock at hot idle,
including the erratic aspect. My best theory was piston slap, but after
driving it that way for several years, I wound up trading the engine off
without ever pulling it apart to discover what the true issue was.
With respect to TeriAnn, I don't see how a valve train problem can get
louder under load, since the load on the valve train does not depend on the
load on the engine. A rod knock, OTOH, gets a LOT louder at low rpm and
heavy load.
Just to cover all possibilities, my current TR3 had a knock that I thought
must be connecting rod, even though it didn't sound or act quite the same.
Turned out the crankshaft snapped through the front web, near the rod
journal. The break was almost invisible with the pan off, the most obvious
sign was the damage to #1 connecting rod where the crank had been flexing.
But once I knew what to look for, I could see the dark line around the crank
web.
But it didn't come and go as David's knock does, so I don't think that is
likely to be his problem.
-- Randall
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