what is the adjectival form of the word "noun?"
You already had the answer in your search term - noun will normally do fine as an adjective.
Consider, for example, a noun phrase, aka nominal phrase. So far as I'm concerned, noun there is an example of adjectivalisation (it's effectively being used as an adjective, the same as nominal).
In certain contexts you might use noun-like, but I'd avoid nouny, nounish, etc.
Dictionary.com and theFreeDictionary.com lists the adjectival form as nounal, however, FumbleFinger's nominal works very well, too.
Noun´al
Of or pertaining to a noun.
Verbs which in whole or in part have shed their old nounal coat.
— Earle.source: theFreeDictionary.com