If you are hearing a hissing sound coming from your toilet it's for sure the result of trickling in the via the supply line. At first, turn off the water valve under the toilet tank and flush the system. Check whether it needs any type of adjustment or something to get inserted which gets removed by default. If after making the efforts you are not getting the real cause of trickling then you should ask the professionals. You can make us call for free and speak with our experts’. After receiving your query we give you an instant reply as per the requirement of your toilet issue.