What does it mean when there is a progressively loud sound coming from the rear of the vehicle?
Depends on the sound - if it changes frequency pitch when you
speed up or slow down, then it's likely the main rear-end bearing
wearing out. It sound like a steady whine - the key is whether the
sound changes when you speed up/slow down, telling you it's a rear
end problem. Brake sounds would only be heard during braking,
etc.
The other likely cause is in the same area, and that's an axle
bearing wearing out. Same sound as a differential (rear-end)
bearing going bad, but less volume.