When I'm driving I get this weird howl or humming sound coming from the rear of the car at around 50mph and it continues/gets louder the faster I go. I know it can't be my tires because it's only just recently started making that noise.

However, I first started noticing it a couple weeks ago...around the time after I painted my brake brackets red. I put everything back together correctly and torqued the lugs to 100 ft/lbs. It seems more than a coincidence that the noise happening around the time I painted the brackets but I have no clue what I messed up if anything that would make that noise.
I got around to opening up the differential and changing the oil.
The magnet had a lot of filings stuck to it and the fluid was really really dark but no broken teeth and the input pinion teeth looked fine.

Afterwards, I took it out for a spin...the humming seemed less pronounced but was still there and what sounded like a lower pitch. I filled it up with Mobil 1 Synthetic 75w-90...I didn't need to add any additive cuz it's the Zexel-Torsen differential, right? Car is a 2001 WS6.

TIA.

 

No one??

C'mon, help a guy out, huh?