Wheel-speed specific vibration under load ('99 M3 Coupe 5spd)

[hardtosee]

Hi all, looking to crowd-source a diagnosis on a subtle but annoying issue with my '99 M3 coupe.

TL;DR is that a vibration under load seems to be wheel-speed specific, and that the car's drivetrain is in otherwise great shape. It's a high mileage car (~140k), but I know the full history of ownership, and the prior owners did not track or abuse the car.

Scenario:

Vibration under load at:

near top of 1st gear (~ 5800-6200 RPM)
around 3500 in 2nd (~ 3200-3500)
around 2200 in 3rd


Felt in "seat"
Possibly a rhythmic "friction" noticed at lower speeds, but difficult to say


Details/History since my ownership:


'99 M3 5spd TitanSilber/Dove, 145k miles
Since purchase/within last 4k miles, I did a significant overhaul of the drivetrain, mostly OEM+ components:

Clutch assembly (Sachs OEM)
Transmission detents
Shifter rebuild with OEM components (bushings/mounts/hardware)
Motor mounts & transmission mounts (OEM)
Guibo/flexdisc
CSB
Slave cylinder + SS line + restrictor valve delete


Also some other goodies, namely:

Wilwood Superlite 6R BBK (140-8797-R) (fronts)
AKG oil pan baffle + M50 drilled oil pump nut
"Lifetime" RSMs



Given the recent maintenance, and the correlation of the engine speeds/gear combos where vibration is noticed to the actual gear ratios of the ZF transmission (http://www.bokchoys.com/differential/GearRatios.htm) in this car, I'm leaning towards this being an issue with something downstream of the output shaft of the transmission. See table below for more details.



Gear
Vibration start, observed
Vibration end, observed
Gear Ratio
Gear ratio, N:N-1
Expected RPM range of vibration, if based on gear ratios


1st gear
5800
6250
4.2
N/A
N/A



2nd gear
3250
3500
2.49
0.5928571429
3438.571429
3705.357143


3rd gear
2150
2300
1.66
0.6666666667
2166.666667
2333.333333



Speaking with a friend, we suspect it could be one of the following:

Improperly mounted/secured guibo
Driveshaft failure (U-joints, or warped?)
Diff clutch failure


Visual inspection of the guibo shows no signs that #1 is the issue, nor do the drifeshaft U-joints appear visually worn or failing.

The car drives well otherwise, but I'm setting it up for light track duty, and it's also just plain annoying. Before I go and get my differential rebuilt, looking for any personal experience people may have had that's similar, or (hopefully not the case) that I should start questioning whether this could be something internal to the transmission itself.

Welcome any and all feedback/thoughts. Enjoy your drive.

As an addendum, here's a picture from when I bought it (because everyone loves a picture!)
675958

[gdavid]

Did you have this vibration shortly after your drive train work or was it more recent? It could be as simple as a wheel out of balance or bent rim. I assume you have stock, staggered wheels and cannot try rotating between the fronts and backs. Does it change in a turn?

[hardtosee]

@gdavid thanks for the thoughtful questions:

- I can't say that I noticed it immediately after the drivetrain work. I suspect it's slowly getting more pronounced.

- I did have a speed-specific vibration (highway speeds) that was resolved with new tires, at which point I had a four wheel alignment and balancing done. Correct that it's on stock/staggered wheels, although I do have 15mm spacers up front to clear the Wilwood calipers.

- I don't believe it makes any difference whether I'm straight line or turning, but I will test this next time I'm in the car.

[hardtosee]

Come to think of it though, I have been slowly and consistently losing pressure in my LR tire.

[Eric93se]

Did you add pre load to the CSB when installing?

I would inspect the lower outer ball joint.

[hardtosee]

Did you add pre load to the CSB when installing?

I would inspect the lower outer ball joint.

I'll confirm with my mechanic that the CSB was pre-loaded. Are you suggesting that, if not, it could cause the ball joints in the trailing arms to go bad?

The fact that the vibration is only apparent under engine load, and that it's very specific to wheel speed makes me question the issue being with suspension components.. but the CSB and/or driveshaft misalignment does seem like a prime suspect. Thanks for the input!

[Eric93se]

No, they are separate issues.