No, it's not the beginning of an old prank-call routine. It's a common homeowner dilemma that's annoying, but easily remedied.
I found this out the hard way, when I neglected that loud noise coming from the basement toilet, and was shocked to see that my water bill was about 15 times the usual price -- almost $700! Oops.
Turns out it was a very simple and common problem: the flush valve wasn't closing completely due to a misaligned flapper
(that black or red rubber stopper thingy, attached to the arm chain, that opens to let water from the tank into the bowl). I fixed it in a few seconds by realigning the flapper with my hand.
Running toilets can waste 2 to 6 gallons of water per minute (an average new toilet uses just 1.6 gallons per flush). That can add up to thousands of gallons of natural, and financial, resources down the drain.
So if you suspect your toilet is running -- and if you hear a persistent sound of running water coming from the tank, it probably is -- here's what to do:
To diagnose your problem, take the tank lid off, flush, observe how the basic mechanisms work, and watch the cycle. If water keeps rushing into the toilet bowl and the tank is not refilling afterward, the flush valve is not sealing properly.
Turn off the water supply at the toilet's shut-off valve
(turn that silver knob where the toilet attaches to the wall), flush the toilet and watch how flapper settles on the valve opening. If it gets stuck in the open position, reach in and close it (don't worry about the tank water, it's clean). If that doesn't work, make sure it's not catching on the flush arm chain.
If the flapper is still not sealing the flush valve, it may be old, crusty, and need to be replaced. Flappers are available at any hardware store. To replace it, shut off the supply line by turning the shut-off valve, drain the tank, pop the old one off, and disconnect it from the flush arm. Then put the new one on and reconnect it. Make sure it aligns properly with the opening, and adjust the chain so that you have about an inch of slack when the flapper is down.
To test it, flush the toilet one last time to make sure the flapper creates a watertight seal with the flush valve.