how can I fix my Isar HE30?

Hi,my three year old Isar HE30 (that has also been a real pain for three years) has today started making a very loud vibrating noise that shudders through the house and is really scary,the heating was on and we used the hot water tap when it started,my husband has checked the water pressure and that seems fine, LF came up on the screen and the book says flame faliure - any suggestions please thanks Sue