Is the head "noun" or "adjunct", and which is the correct plural form? "nouns adjunct", "noun adjuncts", or "nouns adjuncts"?
"nouns adjunct" sounds the most correct to my ears.
An adjunct is a thing. There are various kinds of adjuncts; for example, there are adverbial adjuncts as well.
In linguistics, an adjunct is an optional, or structurally dispensable, part of a sentence, clause, or phrase that, when removed, will not affect the remainder of the sentence except to discard from it some auxiliary information.
The footnote to that definition says it derives from Introduction to Theoretical Linguistics by John Lyons, 1968.
This particular kind of adjunct is a noun adjunct. So it’s one noun adjunct, two noun adjuncts. From Wikipedia:
Noun adjuncts were traditionally mostly singular (e.g. "trouser press") except when there were lexical restrictions (e.g. "arms race"), but there is a recent trend towards more use of plural ones, especially in UK English.
If I invited you to my house boat, I could also invite you to my boat without changing the structure of my invitation. So in house boat, the house part is the adjunct that attaches to the noun boat, but this is the structurally dispensable part. It also happens to be a noun itself, but you can dispense with it if you must, and nothing changes structurally, only some extra information is lost.