Ok...interesting summary to this mess (and yes, I think I solved the 'unknown' issue of the knocking). Kind of long but something that may come of use to someone else down the road.
Years ago, there was a similar unknown issue on Volvo redblocks with a certain bolt coming loose, clipping the harmonic dampener and shearing off. I did a test where I torqued 5 bolts in the same area to 120 ft lbs. After 600 miles of driving, all were still locked tight except the one that was known to loosen you could break loose by hand with a ratchet. I believed it to be (and still do) an issue of harmonics. Loctite on that one bolt stopped the issue.
So now with this car, I sorta thought the same thing but had no smoking gun. Couldn't get rid of the knock, so decided to tear it down and separate the halves to see if there was any damage to the rods or bearings. What I found was....nothing. Bearings showed wear but in perfect condition. A friend of mine who retired from wrenching, asked if I thought I might have a broken valve spring. So while I checked out the head, I was playing with the rocker rails to check for gap and noticed the intake rail moving when I pushed on it. I watched the machinist torque these to spec. Every bolt with a red line was 3-1/2 turns out......this from idling for 10min in my driveway. All the rest are still rock solid, except for those 4 (which matches the knock area that I had and even Subaru refers to). So now I'm really suspecting harmonics.
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When I told the machinist what I found, he said a guy he knows that had the same knock issue so they pulled the cover and......the first 3 bolts were all backed out.
Since I did split my block open, at minimum the bores would need deglazed and re-ringed anyways. So in for a penny, in for a pound.....I'm this far its getting all new bearings, rotating seals, gaskets and o-rings (most of which I found was dried out and rock hard. Wasn't planned but is what it is.....and without the knock.
16mms I think
