Today, I adjusted the valves three times. I had done this a few months ago, and maybe that's when I started noticing the noise getting louder. So this evening I purposefully adjusted too tightly, ran it for a few minutes to see if there was any change. This may have caused it to get louder.
I let it cool down, then adjusted just the first two valves to be too loose. This might have quieted it down just a bit, but it's hard to say. I try to dismiss my bias by listening in different circumstances, etc...
After letting it cool down again, I adjusted all of the valves with a 0.010 blade to where I thought it was probably just a bit too tight, then measured with 0.008 to check, and I think I got some more consistent results.
The engine noise at idle is a tiny bit louder than it has been, but it is louder exactly as expected -- with a bit more diesel clatter. It's a nice, even pitter-patter. It sounds just like I think a 300Tdi should. But the ringing sound did not go away. I think it may be diminished a bit, back to the levels it was over the fall and winter.
Since it is 11:30 at night, I only ran it for a few minutes, then drove it about a 1/4 mile just to spin the turbo up, and make sure the temp gauge moved a bit. The knocking noise did seem to be a little more under control after the last valve adjustment. It wasn't as harsh.
I don't think this fixed the issue. But I think maybe having the valves adjusted better might have minimized it enough that I can drive it to Ship's. I might drive it to work in the morning to get a fresh perspective on what I'm hearing, since I think it is louder once warmed up.
I think it is piston slap. Gee, I hope my wife's Merc gets out of the shop soon!