Sound problem

 I bought an Orion Pal/Secam TV in France and brought it home to England. It worked with no problems in France on Secam. In England I have changed over to Pal and have found all the terrestrial channels ok but there is a problem with the sound. All the time there is a shhh-ing noise as if it is off-channel but fine tuning has no effect. This makes it unwatchable. Please can anyone help? Maggie