I was learning to chant Tehillim in Israel last year. The Syrians have a tradition for how to chant psalms and they base their tradition off the Aleppo Codex.
In their siddurim, Halleluyah is written out as: "הַלְלוּיָהּ" and I have noticed this to be the case in most other Siddurim.
However, in the Aleppo Codex, it is written out as: "הַלֲלוּיָהּ"
Why the difference between the Shewa Na' and the Hataf Patah? I know the Hataf is usually placed under gutteral letters, but that does not seem to be the case here. Why do all of our siddurim show the shewa and not the hataf patah?