2002 Fiesta

Engine whining
2002 Fiesta Manual 40000 miles

My 2002 (52 reg) Fiesta has just started making a whining noise when I drive over 73mph. If I slow down below 70mph the noise stops. It does not happen all the time. At first I thought it might be the road surface, but I doubt that now as it has happened on various road surfaces. Is the noise dangerous and what should I do to fix it? I bought the car just over a month ago and my 28 day warranty has just run out. Thanks in advance for your help.