Join Date: Aug 2016
Location: West Coast Central Scotland
62 MPH to 68 MPH...and you have covered the obvious noises like tyre road noise and wheel bearing noise. Is the noise at the back or the front? Sounds like you are concentrating at the back, but noises can transmit anywhere in a car and not easy to detect the location.
A humming noise refers more to a rotational but can be a vibratory mechanical noise, and all you have left is up front where the engine is, either the front wheel bearings or coming from the engine. Also components under the floor pan vibrating, check exhaust system is secure and not hitting anything, petrol tank secure, brake line fixings secure not vibrating, etc.
With the noise only there between these speeds, it will be caused by a resonance frequency when the car hits these speeds.
A good solution: it might be worthwhile getting a mechanic to test the car stationary on a 'rolling road' to check out for the noise at these speeds. Listening by ear and with head phones attached to a vibration probe for the noise from engine. You have to narrow down where the noise is coming from. Then sort it.
I had a humming noise problem on a Citroen C1 years ago, and it was a faulty component in the engine causing resonance at exactly 55 MPH. Though the noise was obviously from the engine so diagnosis was quick. Citroen replaced the engine under warranty.