Hello, I think your problem is a bad seal, it is located at the top of the flush unit inside the tank,depending on the model, this seal is normaly replaceable,Turn off tank suply water, there should be a cap on top of flusher, should say how to remove, turn or pry top off, there should be a rubber seal inside near top that is black and about one and a half inchs diam. It may not look bad but replace it, bring seal with you to hardware store to buy replacement, very cheep part, should fix your hissing noise, good luck, one other way, if you have a very old model flusher, is to replace the whole unit, replacements are fairly cheep also, maybe about $12.00 for every thing. Don't FORGET TO TURN OFF WATER VALVE FIRST!