my mini has developed a sound that we have never heard
 

the sound is comparible to a jet engine grumbling. we don't hear the sound in low speed but when in a higher speed 50mph or higher it is very evident. thanks

have you rotated/inspected the tires ?

we just purchased 4 brand new tires 3 months ago. could it be our wheel bearings?

Have you looked at them ? I've seen bad camber wear them and cause noise , yes it could be wheel bearings

Noise
 

Usualy wheel bearings start grumbling around 20 mph.
Get up to the speed you feel safe and when vehicle is making the
noise, swerve hard left and then swerve hard right, Wheel bearing noise
will deminish substantly while doing this. Swerve hard left, loads left wheel bearings, swere hard right, loads right side wheel bearings.