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Our toilet will often go into a mood and start hissing. This will go on until I manually operate the filling mechanism to fill the tank higher.

I suspect that it is somehow not filling enough and then slowly leaking until a point where it just triggers the filling mechanism to create the hiss. However, there is no leak into the bowl, and I do not know why the top of the post (as seen in the video) is spurting water.

Video: Hiss is about 4-seconds in.

Any ideas?