I have this hissing sound coming from the toilet that is persistent. The sound continues even after the tank is filled. It appears that there is no water leak but the sound is now getting louder. This issue started approx a couple of weeks ago. I am at my wits end and needed help. Please advise.

Make sure the water level in the tank is 1/2 " below the overflow tube. Sounds like your ballcock valve is not shutting off. Does the toilet randomly flush? If so, you have a bad flapper. Put some food color in the tank and check the bowl in an hour or two. Let us know what you see.

The toilet does not seem to be randomly flushing. At this time, I have toilet blue w/ bleach in the tank. So, I will not able to experiment until that is totally gone. On the other hand, I will try that in a couple days or so. Will let you know as soon as I can. Thanks for the reply. I remain.

The toilet does not seem to be randomly flushing. At this time, I have toilet blue w/ bleach in the tank. So, I will not able to experiment until that is totally gone. On the other hand, I will try that in a couple days or so. Will let you know as soon as I can. Thanks for the reply. I remain.
Yes, that will confuse things. Lets do this, turn off the water at the angle stop. Sound stop? If so, you can refer to my original post.