Ideas on what's causing my click/tick/rattle noise?

Since this issue is with my slant I'm posting it here, but it could well be about any engine. Sorry for the long post but I'm trying to give as much background and info as possible to figure out my problem.

I rebuilt the slant in my 82 D150. It was fully machined by a reputable shop that does know slants. I did the assembly using Doug Dutra's book and my FSM. I watched and helped as @j par put together another of my slants, but this is the first engine I've built by myself so operator error is certainly a possibility. Other than the alternator, everything used on reassembly was brand new parts.

Once built it fired right up on a run stand for the break-in with no issues. All was well with no leaks so I took it home and installed it in the truck. During the break-in we were working with open headers and hearing protection so if my mystery noise was present it would have been impossible to hear.

Fast forward to test driving and further break-in. The truck has no carpet at all just bare metal floors, and the shifter tunnel was set in place with two screws since I knew vinyl flooring was going in soon enough. However, that makes for a pretty noisy truck to drive which made this clicking problem harder to notice and hear.

After 200 fairly gentle miles I re-lashed the valves. I knew I had a loud rattle which turned out to be from the clutch linkage bar going to the throw-out arm. It was too loose making a ton of noise so I snugged it up and that noise went away. However, there was still a noise, best described as a ticking/clicking, like a lifter out of adjustment. Lashing the valves got them back in spec.

After driving it another 500 miles or so, I still have the clicking/ticking noise and I can't figure out what it is. Here is what I've checked so far at just over 700 miles:

I just changed the break-in oil and filter for regular stuff, checked the timing, changed the plugs, and fixed a valve cover leak.

I lashed the valves again after calling OCG to verify the lash. This was done with the engine hot and running.

I re-checked the head bolt torque, it was good.

The noise is RPM related. At idle you can hear it, and the faster the engine runs, the faster it clicks. When I get to 2000 RPM or more, the noise becomes noticeable louder and more metallic sounding.

The noise does not appear to be gearbox or drivetrain related. It will make the noise sitting still in neutral with clutch in or clutch out.

When assembling the engine, I checked all the torque two times for the main and rod bolts, doing so in 1/3 total torque increments. I don't think I messed that up but I guess anything is possible.

At the suggestion of a car buddy, I sent in an oil sample for analysis but it's not done at the lab yet so results are forthcoming.

To further complicate matters, I only have one ear that works (and that one is mediocre at best) which makes discerning the location or direction of sound very difficult. I can sometimes figure it out by which direction is louder or softer, but for me trying to figure this out while the engine is running is nigh on impossible. I have borrowed several pairs of knowledgeable car-guy ears to help find the sound. Some think it's in the valve cover, some the block, and another thinks it's something external to the engine.

If I have to pull the engine and go through it again I will, but first I want to make damn sure it's not something stupid and simple external to the motor. I'd be pretty pissed to pull the motor, disassemble and reassemble, and still have the noise when done because it's something else.

I have the truck back on the lift to investigate from the underside. I'm going out there today to look for anything that could be loose and rattling when it picks up the right harmonic frequency. I'm calling over a buddy with good ears to take another shot at finding where this is coming from.

All that long-winded preamble is to ask if anyone has any suggestions on what could be causing this, and what to check external to the engine. If so, please feel free to chime in.

As RRR says, thank you and drive through!