It is always difficult to join an item like this as all what might be adviced or suggested has perhaps already been tried out. Ok so be it, In my case be sure I only hope it can be of any help.
Did the sound started just after the rebuilding of the head or were you able to drive the car a periode of time without that sound?
Have there been mounted any higher compression pistons or was the head perhaps shaved extensively?
I had this once with the engine of my 1300cc 101 giulietta TI wherein I had mounted higher compression pistons from a 105 1300 junior gt. ( this way the compression would change from 8.5 to 9 if I remember well)
I thought, in all my naivety,that those slightly higher pistons couldn’t do any harm. I tested the engine by turning the crankshaft by hand and it felt perfect and when I first started the finished engine it iddled so nice and I remember that I loved that first sound. But once the engine reached a certain higher rev, I don’t remember anymore how high, I had the same problems as yours.
I found out what was the sound by looking into the spark plug hole with a torch where I could see the traces on the new pistons where they had touched the head ( edge of the dome).
The cure was to get the pistons out again and I had to get them shaved at the outside edge.
Now the engine works perfect for years already and gives me all the pleasure I need, the little devil !
This is one possibility that you could check easily.
If this all is not the case, my first search would be to determine whether the sound comes from the outside or the inside of the engine.
Outside of the engine:
That the sound is there at a specific rev could point to a certain frequency for loose parts, badly fastened parts or parts that touch one to another only at a certain frequency. ( f. i. striking of the oil filter, dinamo, starter , air filter housing or inlet manifold etc. against the chassis ?)
What about the parts related to the belt: dinamo, waterpump? You could check the running of the engine without any tension on the belt for the elimination of those parts?
In most of the cases the sound of an inside failure can be recognised as being more agressive, sharp,dangerous, raw, metal to metal…
Of course I don’t know what you have checked with the stethoscope but if I may say, I’m sure that a good ( extensive) stethoscopic researche will give the most chance to locate ,the easy way, any trouble or at least will eliminate some parts of being the trouble maker if on their location the sound seems softer or sharper.
Finally and in most of the cases , it gives a definitive idea whether the problem is at the top or at the bottom , at the back or at the front of the engine, at the first or at the fourth cilinder, at the inlet or the exhaust side. I think it is important to know in which area the sound is the sharpest. It is always easier to dismantle only a head than dismantling a complete engine .
You can also use the stethoscope at every part and accessoire of the engine to eliminate all of these parts. Even the gearbox if you want.
I definitaly would prefer using the stethoscope, or a big screw driver as correctly mentioned by someone else before , for another hour or even more if needed than having to take the engine out of the car.
Does the oil pressure change on the gauge when the sound occurs?
Is there any play on the crank pulley turning it forwards and backwards by hand?
If the sound is stronger at the bottom, I would suspect the bearings on the crankshaft. Sometimes they just start to fail without any loss of material but with some play at certain revs.
If I remember good, you can take away the oil pan to check the crank and rod bearings and their caps (+ bolts) on any play. I have to admit that I would choose this as my last possibility to find the source of the sound before taking the engine out of the car as it is not the most pleasant job but again better that than having to take out the complete engine and finding out later that it wasn’t a crank problem at all.
Finally I would always have some ‘second opinions’ before starting dismantling the engine just like writing an important letter and having it checked on errors by another person.
This all is only to tell you how I would approach this problem.
I hope this can help a little.