How can I stop my Geberit flushing system from hissing?
The flapper might move a little to the side when flushing, so you fix it it manually. Have the same thing with my TOTO 1.4 gpf. Changed the flapper several months ago, the hissing stopped for a while, now started again. I washed the flapper with lemon juice and soap to remove any calcium carbonate that might accumulate - it reduced the frequency of the hissing but didn't solve it completely. I guess it is the flapper/flush opening contact that got lousy. Maybe a thin gasket would do the trick. Tell you when I check.

Good luck!