Sounds like toilet is running constantly but the toilets aren't running. Even after I turn the water valve off for the entire house, the sound is still heard. This has been happening for a few days now. What could this be? And why is it happening? Thank you:confused:

I've recently joined and I'm not an expert. I think an expert would ask you for more information as well. I had something similar that turned out to be a leak. When you say you "turned off the water for the whole house" there could still be water in the system that can make a sound. Toilet tanks have water in them still. Also is the valve after say a pressure tank if you have well water. I would start by turning off the feeder valves to each toilet and see if that makes a difference especially after turning off the main valve. A toilet could be running and you don't notice it. When you lift the top cover, is the water up to the overfill tube ? The flapper valve or it's seat could be dirty or worn out and allow ater to trickle out. Flush to empty all toilet retaining tanks and see if that sheds any light.
If they are shut off and empty then the toilets can't be the reason.

Great answer from saffle. Let me add a bit to it. Check the tank water level, it should be at the mark in the tank or 3/8 to 1/2" below the top of the over flow tube. Now look at the flapper, If it has wavey edges on the flat surface, replace it. The linkage between the flapper and the flush lever should have 1/4" play when the tank is filled. Also, check the small tube coming out of the top of the bakkcock and make sure it's connect to and draining into the white overflow tube. Good luck, Tom

You may have an underground leak that the sound is resonating through the pipes. When you turn the water off, do you have a valve inside or do you do the meter valve?
I had a call once that the people could clearly hear water running but had no other evidence. I found a broken main at the neighbor's and that solved it.