According to Appendix E of The Lord of the Rings, this has the same
relation to ⟨y⟩ ([j]) as ⟨hw⟩ ([ʍ]) does to ⟨w⟩ ([w]) – this probably
means the voiceless palatal fricative [ç], though Wikipedia says a
voicless palatal approximant (which would be closer to [j], the voiced
palatal approximant) is sometimes also posited.

We previously emitted [[hj]] for ⟨hy⟩, which sounds fairly close to [ç],
similar to how [[hw]] is fairly close to [ʍ] (see previous commit) –
however, translating it into [[C]] again means better --ipa output.

(In Sindarin, ⟨hy⟩ does not occur.)