It might be a blown or leaking damper/shock absorber (look for signs of damp on the damper body/shaft), check the suspension springs, esp. the rear haven't snapped. Prop shaft is a possibility given it is speed related or drive shaft as others have said. Less likely as you state it can't be felt in the steering nor under effected by braking, but warped rear discs.
Gearbox/Rear Diff are a possibility but i would expect a change between the engine being under load and coasting.
I would say damper given it is triggered by road surface (the vibration would be non damped road wheel which is bouncing about at speed) , but prop or drive shaft if it is directly speed related (although out of balance drive shaft or prop, prop shaft centre bearing would always give a degree of vibration, but will have a specific road speed were is it very bad) Given that you can't feel it directly via the steering, I would say rear components (Springs, Dampers, Driver Shafts, Disc and Prop shaft) are more likely as they won't be felt as much in the steering.
A
Grok had this comment:
The comment offers a thoughtful and mechanically grounded perspective on the vibration issue.
1. **Blown or Leaking Damper/Shock Absorber**
- **Suggestion**: A failed damper could allow a wheel to oscillate undamped, causing vibrations triggered by road undulations. Look for oil leaks on the damper body/shaft.
- **Analysis**: This is plausible in general, as a compromised shock absorber could permit wheel hop (~10–15 Hz, matching your 11 Hz) when excited by road input like longitudinal undulations. However, your van’s 2023 vintage and 22,000 km mileage make significant wear or failure less likely under normal use. The comment ties the vibration to an undamped wheel “bouncing at speed,” which aligns with the frequency and road sensitivity. The lack of steering feedback supports a rear damper issue over a front one.
- **Opinion**: Reasonable but less likely given the vehicle’s age. A manufacturing defect or early failure isn’t impossible, though, and the road-specific trigger strengthens this case slightly.
2. **Suspension Springs (Especially Rear)**
- **Suggestion**: Check for snapped springs, particularly at the rear.
- **Analysis**: A broken spring could alter suspension dynamics, potentially causing vibrations, but this typically results in uneven ride height, noise (e.g., clunking), or handling issues—not a consistent 11 Hz vibration tied to 90 km/h. At 22,000 km, a snapped spring is improbable unless damaged by an external factor (e.g., impact). The frequency match to wheel speed (10.8 Hz) also suggests a rotating component over a static spring issue.
- **Opinion**: Unlikely given the mileage and symptom specificity. Worth a quick visual check, but not a primary suspect.
3. **Propshaft**
- **Suggestion**: Possible since the vibration is speed-related; could be an imbalance or center bearing issue, worse at a specific speed.
- **Analysis**: The propshaft in your 4Motion rotates at ~37.8 Hz (assuming a 3.5:1 final drive ratio), far above 11 Hz, making direct imbalance unlikely to match your vibration frequency. A faulty center bearing or universal joint might induce lower-frequency chassis vibrations, but these would typically be present across a wider speed range, not just 90 km/h, and might produce noise (e.g., whining). The comment notes constant vibration from an out-of-balance propshaft, which contrasts with your road-specific symptoms (minimal on flat roads).
- **Opinion**: Possible but less likely due to the frequency mismatch. A secondary check if other causes are ruled out.
4. **Driveshafts**
- **Suggestion**: Driveshafts are a possibility, with imbalance or wear causing speed-related vibrations, potentially worse at a specific speed.
- **Analysis**: Driveshafts rotate at 10.8 Hz at 90 km/h, nearly identical to your 11 Hz, making them a strong fit. A minor defect (e.g., CV joint play or imbalance) could be excited by longitudinal undulations, explaining the road dependency. The comment suggests rear driveshafts are more likely since steering isn’t affected, which aligns with your symptoms (no mention of steering feedback). At 22,000 km, wear is unlikely, but a manufacturing flaw isn’t. Our prior analysis also favored driveshafts.
- **Opinion**: Highly plausible, consistent with frequency, road sensitivity, and 4Motion complexity. A top contender.
5. **Warped Rear Discs**
- **Suggestion**: Less likely since braking and steering aren’t affected, but still possible.
- **Analysis**: Warped brake discs typically cause vibrations felt during braking, often through the pedal or steering, and at a frequency tied to wheel rotation (10.8 Hz). You haven’t reported braking issues, and the vibration’s tie to undulations (not braking) weakens this case. At 22,000 km, disc warping is rare without aggressive use or overheating.
- **Opinion**: Very unlikely given the symptoms and vehicle condition.
6. **Gearbox/Rear Differential**
- **Suggestion**: Possible, but a change in vibration between engine load and coasting would be expected.
- **Analysis**: Gearbox or diff issues (e.g., worn bearings) could produce vibrations, but these typically occur at higher frequencies or with audible symptoms (e.g., whining). The comment rightly notes load/coasting differences, which you haven’t mentioned, and the 11 Hz ties more to wheel speed than internal gearing. At 22,000 km, these components should be sound unless defective.
- **Opinion**: Unlikely without additional symptoms (noise, load variation).
### Synthesis with Our Analysis
The comment prioritizes a blown damper due to road surface triggering, followed by rear drivetrain components (driveshafts, propshaft) due to speed specificity and lack of steering feedback. Our prior analysis leaned heavily on driveshafts due to the 10.8 Hz match and 4Motion context, with suspension as a secondary factor. The new details—2023 model, 22,000 km, and longitudinal undulations (not general roughness)—shift the balance:
- **Dampers**: Less likely due to low mileage, but a defective unit could explain undulation sensitivity if it fails to control wheel motion. The 11 Hz matches wheel hop range, though newness weakens this.
- **Driveshafts**: Strongest candidate, aligning with frequency, road-specific excitation, and rear-component focus (no steering feel). A minor flaw could be amplified by undulations.
- **Propshaft**: Possible but less probable due to frequency mismatch and less road-specific behavior.
### Grok's Opinion
The comment is well-reasoned and aligns with automotive diagnostics principles, correctly emphasizing rear components and road interaction. However, given your van’s near-new condition, I’d adjust its weighting:
- **Top Suspect**: **Driveshafts** (rear, given no steering feedback). The 11 Hz matches their rotation, and longitudinal undulations could excite a subtle CV joint or balance issue, even in a new vehicle.
- **Close Second**: **Damper (rear)**. Plausible for undulation sensitivity and 11 Hz wheel hop, but less likely at 22,000 km unless defective from the factory.
- **Lower Priority**: Propshaft (frequency mismatch), springs (no wear evidence), discs (no braking issue), gearbox/diff (no load variation).
