You need to forgive me I am not am not sure what all the parts of the toilet are called so I hope I can explain this well. The other day I noticed my toilet was making a hissing noise constantly. I took the lid off the tank and noticed it is coming from the fill valve area. I noticed there is a little knob on the fill valve that I can turn to lower the water level in the tank. What is strange is that no matter where I adjust the water level in the tank the fill valve allows the water to flow to the top of the overflow tube. For example: If I adjust the water level to stop about 1/4 in. below the overflow tube, the water will fill up into the tank to that point the it slowly allows water to continue entering the tank. Its as if for some reason the floating ball can never get up high enough to cut off the water. Any ideas on what might be causing this and how to fix it?

The float ball lifts on the arm to close off the flow of water into the tank.
What happens if you lift the arm directly? Does it stop the water completely?
Sometimes the rod to the float can be bent so that it allows the water level to go higher than it should.

If the tank fills too high the excess water will flow over the top of the tube and drain so that it does not overflow the tank, do you see that happening?
If it never gets that high then it sounds like it is not overfilling so the flapper at the bottom is probably letting water leak out enough that the toilet has to keep topping itself off to stay full.

If lifting the float arm does not work to shut off the valve then the valve probably needs replacing. If it does shut off the flow of water then perhaps the float arm needs more adjusting than the screw allowed for, perhaps due to a bent rod to the float and it might be possible to bend the arm back down to get the rest of the adjustment.
If the flow of water into the tank stops but the level slowly drops or you can even hear trickling of water then the flapper may need to be replaced.

One of the resident experts can tell you more but I have had to look at these problems so many times in my own homes that the above will most likely cover the problem even if I do not have the terminology right myself. :)

Thanks for the reply. If I were to pull up on the arm directly it stops the water flow. Once I let go you can hear it hissing again. It continues to do this and yes the water does rise above the overflow tube. When I get home tonight I will check to see if the rod is bent. Thanks for the help.

Sometimes the old floats have a leak in them allowing water to get in so they sit lower in the water of the tank. So even if the rod is not bent the float is not lifting as high as it needs to for the fill valve to be closed. Or if I remember correctly some solid floats could over long periods become saturated so they no longer floated as well.

All of these parts are available at the local hardware store or even a WalMart if you had to.

Most fill valves are under 10 bucks and easy to replace. That's what I'd do in this case. Also change the water line from the shutoff to the fill valve while your at it (if you have a shutoff behind the bowl)

Thanks for the help guys! You were right the float actually had some water in it that was keeping it from lift high enough to stop the water. I replaced the float and the problem was solved.