This is how to solve harsh noises like clunking, vibrations throughout the drivetrain, binding of the "center differential" (really, the AWD clutch) and even wheel skipping or hops due to worn out fluid. All of this is mistakenly disregarded at times as "Quattro rumble" and a normal thing for cars with the Quattro Ultra drive system. The solution is to perform the flush procedure outlined in TSB 39 21 43 2059057/4 - 39 20 28 2059057/1.
Featured tools & product:
ATF Fluid Adapters - https://geni.us/atf-filler-adapter-kit (Amazon)
PT Fluid Pump - https://geni.us/PT-fluidpump-w1139 (Amazon)
Clutch (0CJ) Gear Oil - G 055 515 A2
Rear Diff (0B0) Gear Oil - G 060 190 A2
Clutch fill/drain plugs - N90281802
Reference resources:
Audiworld (drivetrain and fluid variations) - https://www.audiworld.com/forums/a4-b9-platform-discussion-212/center-diff-bad-3051820/
TSB - https://static.nhtsa.gov/odi/tsbs/2021/MC-10199368-0001.pdf
About Quattro Ultra (pg. 26 on) - https://www.vdveer-engineering.nl/en/information/vag-ssp/ssp-vag/ssp-vag-en/432-ssp-657-audi-q5-type-fy/viewdocument/432
VCDS Learn values for oil aging & security lockout - https://forums.ross-tech.com/index.php?threads/31679/
Chapters:
00:00 - Symptoms of a failing Quattro Ultra
01:36 - What the TSB is for
04:19 - Talkin' fluids
06:20 - Procedure prep outline
07:56 - DIY TSB process
13:05 - Reflecting on the TSB effectiveness