Alright, sounds like it is time for an update, in case this helps anyone in the future.
Some background for starters:
- Car came with LSD (dealers were installing those pre-sale, and touching/re-boling the drivetrain) .. maybe an important factor for later.
- Ever since I bought the car (not the first owner, but under 30k miles) - there was that weird thud noise any time I transition from acceleration to coasting and back, as described in the first post.
- I installed a short-shifter and poly shifter bushing. That helped with this strange "clunk" a lot, but not for long. More on that later.
- Another observation was related to vibration at speed (above 70 mph). I never connected the two issues and always thought I had wheel balance issues or a bent rim.
Fast forward, vibration issue got worse and all the way to the point that I no longer enjoy driving the car on a highway. Everything is great at slower speeds and when accelerating, but as soon as I let off the throttle I get that thud/clunk and vibration. As soon as I give it even slight throttle - we are back to normal.
Tired of this situation I went for all 4 wheel refurb/straightening and road-force balancing. Two were indeed slightly bent and were redone to 100%. Excited me went on a highway to only discover zero change in experience, ugh...
Next - took it to two shops for inspection as I ran out of ideas on jackstands. Shops came back with the following suggestions: engine mounts, trans mounts, clutch/flywheel, diff clutches, subframe mounts, tires. I mean - can't prove any of this and all seems to be just a shot in the dark. Did not move forward on any of this.
Further reading, I found numerous topics about older F series cars, mostly 5 series with driveshaft issues. They had a different connection to the rear diff than F22 and now having a lot of issues with vibrations due to driveshaft lisalignment.... Well, between that and the idea of broken diff mounts, I got the camera going and tried to just bounce the car in 1st and reverse while standing on the brakes - see the results below. My hope was to see at least something.
My two observations and thoughts (please comment if you see anything else):
1) Clearly, driveshaft is not firmly connected to the diff. You can see it is shaking and loosely moving around while not under load (I guess 3-5 mm play). I did compare it to another F chassis and it should be absolutely solid).
2) Rear diff bushing seems to be moving quite a bit. Going to inspect it further, but did not find any issues yet.
https://youtu.be/kiGn9z_8RQU
What is next?
I ordered a new 50mm insert nut to connect driveshaft to the diff along with 50 mm wrench. Ordered new guibo (mine looks good, but it is factory Aluminum one and BMW superseded it with the rubber one - some say that aluminum may have failed and can't be diagnosed until removed).
Plan is:
-take off the driveshaft (check insert nut torque before removing).
- replace guibo along with 6 new bolts.
- inspect driveshaft central support bearing and splines for wear + re-lube.
- put everything back together and tighten the new insert nut.
- further review diff bushings for damage.
Will post an update when parts arrive.
P.S.
- The fact that dealer touched the rear diff, could this be the reason why driveshaft is loose on the diff? Was something not tightened properly?
- when I installed the shifter and shifter bushing, all this work involved a lot of driveshaft touching as it was very hard to reach those parts... I suspect that I moved it just enough so it was in the right spot to correct the problem for a few miles.