A noun-noun construction is a combination of nouns that are not a compound noun, not a complex noun, and may or may not be lexicalized.
But what is the concrete difference between a compound noun and a noun-noun construction?
I already answered this question in my previous email, which I was about to send when you sent this one.
Imagine that instead of 'Iw HIq we had 'IwHIq and instead of ropyaH we had rop yaH. How would the language be different? Would these words have different a usage, meaning, grammar or pronunciation? Would something else be different, and if so, what? What is the justification to have a distinction between these two ways to form similar word combinations?
I've addressed usage and meaning, and grammatically I've pointed out that the first noun in a noun-noun construction can take a suffix, but the first component of a compound noun cannot. I think {rop yaH} means something very different from {ropyaH}. If I told someone to go to {roplIj yaH}, I might be telling them to go to a leper colony and not an infirmary.
But you raised the point of pronunciation, and I think there's a difference there, too. Okrand wrote that the pronunciation of {wab Do} "speed of sound" and {wabDo} "Mach number" are the same. But I think that the *stress* is different. In {wab Do}, both words have equal stress. In {wabDo}, the {Do} is stressed and the {wab} is not. In {tera'ngan}, the {ra'} is stressed but not the {ngan}. In {tera' ngan}, both the {ra'} and {ngan} are stressed. So the presence or absence of a space serves a purpose, which is to reflect the stress in speaking.