C180 resonating noise at 70mph

[SW11]

hi - my 02 C180 automatic makes a resonating sound when on the motorway when get to the 60-75mph range.

it comes from the back, i'm guessing from the driveshaft, it doesn't make this noise when driving around town or going through gears getting up to 60mph and doesn't do it when go through 75mph. When cruising on the motorway with foot lightly on pedal the noise usually stops and reappears depending on change in revs.

any suggestions as to what this might be?

thanks