I'd look at the input shaft seal for leaks . That could indicate a bearing that is on it's way out.
FWIW changing diff fluid at 25k miles seems excessive. I'd still check the level or even if it's the correct fluid. The best way to confirm is to do a fluid replacement with something better than OEM [I prefer LiquidMoly]

 

I'd look at the input shaft seal for leaks . That could indicate a bearing that is on it's way out.
FWIW changing diff fluid at 25k miles seems excessive.**I'd still check the level or even if it's the correct fluid.**The best way to confirm is to do a fluid replacement with something better than OEM [I prefer LiquidMoly]

No sign of leaks from anywhere on the diff.

With fluids being free through FCP, and swaps being a 5 min affair with my air powered pump... I see no reason not to do 25,000 mile fluid change intervals (which are actually OBC calculated inspection intervals-- not 25,000 miles specifically.

Either way, it's only twice as often as BMW suggests-- so the equivalent of doing 7750 mile oil change intervals.

 

What speed does it occur at? Does it happen when it's in gear and if you put it in neutral and coast?

 

if youre coasting in gear and apply the ever so slightest throttle and noise come on then its likely your differential. I would still take it to shop and run in gear up to speed and it should be easy to locate the noise.

 

Happens at all speeds, tone increase with speed.

No correlation with load at all, just speed.

... driveshaft?

 

I guess I'm not really sure. I am curious what your issue turns out to be though, as I'm having a similar issue but it only occurs around 55 mph. Might just be the winter tires though.

 

I had been ignoring this because I thought it was my bald winter tires. Then I switched to summers on Thursday, and it didn’t go away.

 

I just went through this on my E46, vibration underneath center on acceleration. More forward of yours but bottom line is it's very hard to pinpoint the problem based on what you feel or hear. Is it original shaft/guibo/center support? Common things being common my bet is center support which can be replaced itself and save you $$$. If you wind up needing a new shaft I recommend Driveshaft Specialists in San Antonio. Excellent service.

 

The guibo and CSB were replaced with new OE fairly recently.

I have have a used, good condition shaft en route from Clemster.

 

if it get louder with speed then it likely a bearing some where. ez thing to do it get it to a shop and run it at speed to check the rear wheel bearings, diff, and trans. the front just have someone spin it and compare to other 3.