I’m just a dude from the USA and I use both pronunciations!

Tin-ite-us is my go to but I’m trying to get away from it. It makes it sound like it ends in “-itis” when I say it. This makes it sound like an inflammatory disease, which it isn’t necessarily.

Tinnit-us feels off to say, but it doesn’t imply the “-itis” part, so I’m going for it!