We replaced the flapper but still makes a hissing noise sounds like it comes from the top (I believe it is called the valve) but it runs non-stop and the water in the bowl even gurgles constantly for a long time. My water bill has tripled over the last 6 months... help!

Before you do anything check the water level if it is OK, then you may have problem with the flush valve, close the angle valve flush the toilet now remove the lid from the tank check the seat on the flush valve for any distortion or dirt clean the seat where the flapper sets now try if it leaks or not if still leaks replace the flush valve it is cheap and easy to replace see the image below, good luck.

John