You need to say what year and model you have, and mileage.
Has anything in the driveline or chassis been replaced/repaired recently?
Did the noise start before you replaced the (rear, I assume) wheel bearings?
Have you tried rotating the tires?

Could be the differential. Worn ring & pinion gears tend to have
harmonics that are
strong at certain speeds and manifest as a howl. This is sometimes
attenuated by chassis parts
that are close by, which begin vibrating too, perhaps because they are
loose and/or a rubber
bushing is worn through. Driveshaft noises tend to be loudest when
accelerating or decelerating,
and cyclical when cruising.
Does it get worse the higher you go above 50mph, or does it subside?

If you have a back seat, it might be useful for you to ride there (with
someone
else driving the car at 50mph) and put your ear down to various places
forward
and rearward, left and right, etc. to see if you can pinpoint the source
more closely.