nyron

hi, i have a 1996 honda civic coupe and for the past 2 or 3 months, when i hit about 70mph on the motorway i can hear a loud humming from what sounds like is coming from the rear, about a month ago i replaced the n/s/r wheel bearing, actually the whole hub, as only comes complete with hub, and was still making the noise, then decided that it might be coming from the o/s/r, so then i replaced the offside hub, againg the noise is still there, someone suggested swapping the tyres from the front to the back, to see if that would make a difference, but again, can still hear the humming noise, it seems to be louder when someone or people are sitting in back, i cannot understand what else this could be, may be perhaps the rear shocks?????
can any one help or advise please ??????