Rhythmic vibration/resonance under light acceleration

[Dantor]

My Z4 (35i with DKG) produces very strong vibrations under acceleration or driving up-hill. This only happens with a low RPM of the engine, so for example driving 90 km/h on the highway and then accelerating to 110 km/h. The vibrations are rhythmic and cause a strong buzz throughout the car (comparable to driving over road markings while changing lanes on a highway). The vibration only happens during light acceleration or driving up-hill on the cruise control, mostly when in the D7 gear. It immediately disappears if I shift down manually or accelerate harder to cause a down-shift. There are no vibrations when driving under normal circumstances (e.g. city traffic or cruise control on the highway).


What do you think it could be? I've had the hardy-disc (flex disc) checked visually and looks fine. Also no errors when reading out the diagnostic log of the car. Further, the rear tires are almost gone but still have about 2-3mm left. I've been to a BMW shop and they did a test drive, confirmed the problem, but have no immediate idea. Before spending way too much on diagnosing, I'd like to see if someone here has an idea.

Could it be:
- worn clutch plates of the DKG?
- wheel bearings?
- rear diff?
- something inside the gearbox itself (as it's only in the 7th gear)?
- bearings of the drife shaft?
- worn rear tires? (but wouldn't that then be a constant vibration, not just when accelerating lightly?)

Many thanks for your expertise here!

Cheers,

[93Siro]

Does it happen when you try to go uphill in a ramp with medium to low throttle input as well? My guess is the clutches.