2004 cadillac cts humming from rear axle or drive shaft area at 30 MPH and lower speeds.

it could be center bearing, if its going bad it can also cause a vibration, make sure you have sufficient fluid in the diff as well, you can also get a noise and vibration from collapsed motor or trans mounts

I seen this before and is the upper rear axle support, or the engine support just open your hood and see if the motor move to much, or get on the back the car and look under to see it the axle move to much wile some one is shifting. MAKE SURE TO APPLY EMERGENCY BREAK

Just about has to be an axle shaft,either bent or has a bad CV
joint. Might br able to see it but it would have to supported at
the lower control arm (so it is at normal ride height,not the weels
hanging unsupported)

Tire replacement is about normal, as that is the CTS-V, it likely has summer only sport tires, those have a very short lifespan.

Do you have another Cadillac dealer in your area, so that you can get a second opinion on the axle locations? That doesn't sound right to me, but then again, I've seen some crazy factory stuff on other makes and models.

Ring and pinion noise for sure,yes. Check for play right at the yoke where the drive shaft goes in.
I've seen a few fords where the pinion nut was loose and after re torquing they lasted another 100,000

I had that on an HHR, the tire tread was coming loose.

you may have one of the cv-axle goes bad. jack one side of a time, put parking brakes on.
start the car and put in drive run up to speed and feel the vibration, the side has more vibration is the one that goes bad

Hey Gary, take your car and get the transmission fluid and filter changed immediately... you will be glad you did...(expensive lessons speaking!!!)

MOST of the hums after a while come from the diff as the back axle does the driving and almost all its life runs the same way except when going in reverse ,is the rear diff meshing and worn on both the crownwheel and the pinion and the humming gets slightly louder the faster you go and almost goes when almost stopped and this can also be the slight vibration you get so get the diff oil drained and refilled and check if there is any metal in the oil when drained if so thats your problem But it will last for years like that..Let me know how you get on RON

Sorry I don't have a solution, just adding that you're not alone...

I just bought my '05 CTS-V on 8/31/09 and today (9/4/09) was the first time I've driven it in the rain. When I hit hard rain it sounded as if the tranny or rearend was going to fall off. I thought I had thrown an axle being or jacked up the tranny/rearend gears or something. The sound quieted down as I left the heavy stuff and then totally disappeared when I got to a dry area. One person said it was probably water hitting the undercarriage or wheel-well, but this was more than that. This sound was louder tan that and is not normal splashing water. If this noise was common, there'd be a heck of a lot more CTS owner complaining about it. I've never heard such a noise in any car I've ever been in when driving in the rain. I'd love to know what it was. I'm not even sure how to show or explain the problem to the dealership in order to get it fixed