1997 Subaru Legacy Rear Rumble

The parts you listed won't cause a rumble. The most likely suspect is a noisy wheel bearing.

There is a tool you might be able to borrow or rent from an auto parts store that borrows them called the "Chassis Ear". It is a set of six microphones, a switch box, and headphones. You clip the microphones to suspect points, then drive around while listening with the headphones. You can move the microphones around to zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool. Suspension and alignment mechanics use it to find rattles, squeaks, and other noises.

A bearing in the differential could be noisy too, but a noisy wheel bearing is much more likely and common. You might be able to run the vehicle in gear on a hoist and listen with a stethoscope, but that doesn't put the bearings under load, and it can be real hard to identify the noisy part. A good bearing and a noisy one will often sound almost the same when not under load.

You can also jack up the rear wheels, then spin one wheel at a time by hand and feel for a vibration in the coil spring. Noise from the differential shouldn't transfer to the spring but you will usually feel a vibration from a noisy wheel bearing.

Be aware too that bearings don't typically cause a rumbling noise. They most often are described as sounding like the buzz of an airplane engine that changes pitch with changes in vehicle speed. The Chassis Ear is the fastest way to locate the source of the noise.

Pinion bearings in the front of the differential can cause a rumbling noise too but those can usually be identified just by standing next to it while it's running on a hoist. Those do not make the same buzzing noise as a wheel bearing.

Depends on what kind of music you're listening to!

When a pinion bearing is that bad, there is usually going to be gear oil leaking from the seal too due to the shaft moving away from the lip of the seal as it wobbles around. Regardless, to be that loud inside, it should be real easy to hear when standing underneath. The severity of the noise is still going to be dependent on load, but an experienced mechanic will recognize the noise with a stethoscope.