from my own experiences, but not universal :
- if you have a broken valve spring, the engine starts very hard and can not operate normally. so you know right away that you need to remove the head.
- if the noise cannot be defined precisely, it is probably a damaged crankshaft bearing. it is possible that the oil is still clean or without visible evidence. If you check all the upper engine, you have to investigate now in the lower engine.
I had this problem on a 1750, the grinding of the crankshaft, made by a specialized workshop, had not been correct.
I had confidence in the workshop, I rebuilt the engine 3 times. the first and second time, crankshaft gone to this workshop because I thought I made a mistake. The third time I changed the workshop, and they found that the grinding of crankshaft was not perfectly made...
Hope you will find the solution in short time.
Chris, in love with Alfa from...'68, but it wasn't always reciprocal !