There are numerous possible sources of humming noises. Wheel bearings are one, the diff is another. Something rubbing on the exhaust can make a very loud hum / whistle. If it's a steady, fairly quiet hum then it's likely something minor. If it's more of a throbbing, grinding hum then it probably wants fixing. I had that on a Mk1 Vitesse when the wheel bearing failed, and it was obvious from a brief inspection because the brake backplate was no longer parallel to the drum!
If it's load dependent (you say it's only at steady speed?) then I'm leaning away from severe bearing failure. Diffs definitely can behave that way and it's often not a problem, but check your oil level.
Current fleet 1967 Vitesse 2L conv "Tessa" 1970 Spitfire Mk3 (project) "Toby" 1972 GT6 Mk3 Not enough time or space
Recently sold: 1972 2.5 PI estate "Harry" (gone to russell on here) 1973 Toledo 2-door "Spike" (gone to a club official)
at a steady speed have a think about tyre noise. does it change if you try adding or removing a good few PSI
1964 1600 Vitesse 6 Cactus and Black , now sold now have T2000 Mk2 saloon in French Blue/grey trim been restored without running since 1997 now has power steering ,poly bushed and Alfa 156 seats location Luton