I have a Speech Pathology colleague (of many years standing) who I have heard several times recently pronounce obstruent as /əbˈstr.../I share Martin’s bemusement. I have never heard anything but initial stress for this word, ˈɒbstruənt.
I was wondering whether this is just because it is a word she had not heard said often (though in this same meeting I had used it several times), or whether this is a recognized US variant (not in LPD).
All other English words ending in -uent have antepenultimate stress for most of us, too: affluent, effluent, congruent, constituent. However, some Americans do say conˈgruent with penultimate stress, and some do the same with affluent and effluent, so perhaps this is what provided the model for the obˈstruent produced by Martin’s colleague. (But I don’t think even Americans ever say constiˈtuent.)
Classicists will know that the Latin etymon has a short vowel, obstrŭ-ō, -ent-, and therefore initial stress.
Words with other vowel stems plus -ant ~ -ent include brilliant, radiant, valiant, variant, gradient, lenient, prurient (all antepenultimate), but on the other hand compliant and defiant (penultimate, because of the verbs comply and defy).
A word I don’t think I’ve ever encountered outside early nineteenth century poetry is reboant. The OED says it has initial stress, ˈrebəʊənt. Merriam-Webster agrees (sound clip).