A sudden continuous water rushing sound coming from what appears to be from the water heater to the outside water pipe. Sounds like a fire hydrant was opened but no water entering the house. We have turned off the water supply valve awaiting plumber. Any suggestions as to what is happening?
First, you should also turn off the gas or electricity to the water heater. It shouldn't be heating if it is leaking or less than full.
Water heaters have a pressure and temperature relieve valve that looks like this:
Sometimes they are on the top, sometimes they are on the side. It should have a pipe coming off of it going... somewhere. Sometimes the pipe is tied into a drain line in the wall, and sometimes it's a separate drain that runs outside and drains on the ground. If the water heater is in the garage, sometimes its not connected to anything (but it should be).
Anyway, when these fail, it normally starts to drip, but if it is plumbed into the house drain, you might not see anything. They normally just drip, but its possible the valve has dripped for some time and has now failed so much that you can hear the water. If that's the case, the pipe connected to it should be hot when water is flowing.
You might not be comfortable changing it yourself, but it's a relatively easy job and an inexpensive part. They can get stuck, and the threads can be damaged and that can lead to replacing the heater, but usually thats not the case. If that is the problem, a competent plumber should have it replaced quickly (should have the part on their truck).