Aha! Found it! Tibetan singing bowls (try a search--there are lots available at non-buddhist prices!)

There is also a kind of whistle, used in orchestral music for bird calls, which has water in it somewhere; and the water moves down the pipe as you blow, producing a swooping note with a bubbling effect (like some birds' songs) which is very difficult to describe. And isn't what you're looking for anyway!

Steve