Pretty certain the owner has been filling up with renewable at 76. But that wouldn't mean much unless they were all uniform in behavior. I believe a new timing chain was installed as well, at least he said he was going to and it's logical to do that during the head swap.

Throwing any ideas out no matter how unlikely they may be...
I figure you're pretty certain it's coming from #1. My work wagon was making a sound like nailing. Ended up not being the injector, but a worn intermediate shaft bearing and the rollercoaster follower was kicking the plunger inside the vac pump enough to blow the bearings and snap the piston/plunger(probably not good for the IP on the other end). Shot in the dark, but it's something to rule out at least. And my rebuilt injectors in my 77 300D were kinda loud at first, though they were uniform(I assumed). At 106K the chain is not a factor, though I did adjust valves for the second time in 1,000 miles as they were both tight and loose when I bought it at 104K, and also pump timing a bit to quiet things down and fix the smoke. Sound is pretty much "gone" now, 500 miles later.