Ford Territory has droning noise

My 2010 Ford Territory has developed a droning noise when traveling at 78 to 82 km/h that has steadily become more audible. My Ford dealer diagnosed the problem as a transmission fault and replaced the auto trans, but the drone persists as before which suggests that the transmission was not at fault. My dealer now says the noise is due to converter lock-up and it is normal, but if it worsens they will investigate further. What do you think?