My 2017 Holden Colorado RG 2.8 has a resonating vibration in the cabin while driving that goes away at higher speed. There is a slight engine vibration felt through the steering wheel at idle. My mechanic says it’s not the auto transmission. A valve-body upgrade made no difference. The mileage is now 136,000km. There are no fault codes showing, but the problem is getting worse over time. There’s also very slight movement in the forward driveshaft uni joint. What could be causing this?
You may have a collection of problems rather than just a single one. The vibration through the steering wheel could simply be the vibrations from the diesel engine travelling up the steering column and into your hands. Does the vibration go away when you rev the engine to just off idle? If so, perhaps an adjustment to the idle speed is all that is required there to move the engine out of its resonant patch.
A vibration in the cabin could be a worn wheel bearing, out-of-balance driveline, a worn torque converter, broken or worn engine mount, broken exhaust mount, damaged tyre, a wheel out of balance and plenty more. But the play in the front driveshaft coupling would be a smart place to start looking. Again, the problem comes and goes because of the harmonic periods inherent in things that spin very quickly. Any out of balance problems can easily show up as transient vibrations inside the car.