2007 bmw 550i 97k rear diff question
 

is that an oil you need to change? i don't want to say it howls though my bad/bald tires were making it howl liek that, changed them and still does it and was wondering if that way typical? i bought good tires mich. pilot a/s3's . car has 97k and if it is on the maintanence side and needs to be done i'd do it (any DYI's would be great)

Your rear diff does have oil in it but I highly doubt the noise is coming from the differential unless you notice any leak in the center of the rear axle. Then maybe your diff is low on oil. I'd first take a look at the brake assembly, maybe something is binding. Does the noise only happen when driving/moving or is it constantly howling even at idling?

I had Michelin Sport A3 on my 550 and they were noisey enough to make me think it had bad bearings but the type of pavement made the noise change pitch so I knew it was not bearings or rear diff gear noise. Does your noise change on smooth versus coarse pavement?

No leaks but i'll look at it again. More speed related anything over 20mph i hear it and louder when faster...which makes me think tires. I'll play with tire pressure and see if it helps

Sounds like tires or bearings. Can you upload a video of the sound at the speed its most noticeable?
What kind of tires do you have and are they wearing evenly? If the tires have less than 5k miles on them its unlikely they worn unevenly already.
If you have a staggered set up the back tires are bound to wear down quicker than the fronts, which doesn't matter. It only matters if the front right tire is significantly more worn than the front left or vice versa. Same applies to the back.

I'm placing my bets on the wheel bearings or brakes.

EDIT: Typically worn wheel bearings will allow more looseness in the wheels. A good way to check for worn bearings is by jacking the car up and attempting to wiggle the wheel. It should be very stiff and not have much play - if it moves side to side that's a sure sign of a worn bearing.

My guess is worn rear differential components. Had something similar happen on my 1985 MB 190e. Tire shop lifted the rear by the rear diff. when it was fairly new and broke the seal causing the lubricant to leak out. Noticed it eventually and had the seal and lubricant replaced under warranty however at about 120k miles a whirring could be heard from the rear that eventually grew much louder over time. My indie mechanic eventually rebuilt the differential, cost was about $2k.

My car is bone dry...with only 97k are bearings that much of a problem?

i changed my diff fluid at about 90k miles, and the old fluid came out literally looking clear like new.
one way you can tell if the noise is from the drive axle or diff, does the noise go away or change when you let off the throttle? if yes, then noise is driveline related. if no, something else.

No when letting off sound just slows down with speed and when on gas louder when speeding up...

well, then i'd lean towards tires or wheel bearings. sometimes you can tell its the wheel bearings by doing some slight turns at speed, which will change the loading on the bearings, and noise changes.