VERY URGENT ! central heating problem

Somebody please help, I have a problem with my central heating pipes, even though the heating is turned off there is a constant hissing coming off the pipes, its terribly loud and when i listen to the pipes there seems to be a flow of some sort, one of the pipes seem warm and when i listen to the pipes to every one of the radiators i can hear this loud hissing sound, whether it be gas or water i havent a clue, i have absolutely no diy experience and am very worried about whats going on, I know it cost loads to get someone out so if i can get any help whatsoever i would be most thankful
Please help.

The only description of what you've got in your house is some pipes which hiss.
That isn't much to go on - we need details of the system.
Just guessing, it might be that you have a combi and the pressure relief pipe is letting water out (outside) because the filling loop is still connected and not turned off.