By what definition of "hypnotic"?
From some of the definitions here, which seem to focus on "stuff that makes you feel fuzzy and cozy", then I'd say the rain is a big one for me. The sound of the rain is one of the prettiest things to listen to, good not only for deep relaxation, but even for mild one, like thinking calmly about something.
Showers are also nice for the same reason; warm and soft whispers too, and anything that involves "background noise", which seems to be a common pattern.
Another pattern is "feeling in a vacuum", be it by looking at the sky (I love grey, homogenous skies in particular), being immersed by water, or anything of the like.
Where I live birds come by a lot, and I love listening to the various sounds they make, it's very nice for some reason.
And I second that pendulums, spirals and weird flashy things with leds are more annoying than mesmerizing.