V40 Noise at speed

I have a 1999 V40 1.8 which when travelling below 80mph is very quiet and little road noise is evident but between 80 and 90mph the cabin noise increases dramatically, it is best descibed as a low frequency background noise. I have fitted new tyres which has improved it slightly, the balancing is OK and no vibration is felt through the steering wheel. I have been told this is normal for volvos built during this period and that mine is quiet compared to some. Does anyone else have this problem with their V40's? Thanks Cris