Howling in rear end

Tires and bearings are both noise sources. Worn bearings will make a growling sound as you drive which will usually change in level if you swerve the vehicle back and forth. The swerving changes the loading on the bearings and the noise changes. For tires, if they wear in a strange pattern or if they are molded with large tread blocks they can howl. An easy way to discover if the sound is from the tires is to listen to them as the road surface changes. If you are driving along at 55 mph and hear the noise and the road surface changes from asphalt to concrete or even to new asphalt the tire noise should change as well. If you have a dirt road nearby and drive it and still hear a howl it is not the tires.
Another testing method involves a tool called a Chassis Ear. It is a set of vibration sensors that you clamp to possible sources of noise, then you listen to the sensors as the vehicle is driven. You can select each sensor and pinpoint the noise by listening for the loudest area of noise.