Drone and vibration in my 2014 Isuzu MU-X while highway driving

It’s difficult to be definitive here without actually driving the vehicle, but there are at least two possible sources for this. The first is the simplest and involves either a wheel and tyre or driveshaft that is out of balance. That should be easy to diagnose and fix.

The second possibility is that the vehicle’s driveline (engine, gearbox and transfer-case) has what’s called a resonant frequency that, when reached at a particular speed range, will result in some weird harmonics and vibrations which is possibly what you’re feeling and hearing. This becomes worse when the torque converter locks up (to save fuel) and there’s no slip in the driveline to absorb these bad vibes. By shifting back to fifth gear, you’re raising the engine revs and moving it out of that harmonic patch.

I’ve heard that Isuzu has been remapping some ECUs (on-board computers) to try to move this harmonic zone into a different (higher) speed range (by manipulating the torque-converter lock-up point) where it won’t be a problem to most owners. The fact is the Isuzu diesel has never been the most refined one out there and some vibrations and harmonics are relatively common, However, they shouldn’t be bad enough to ruin your enjoyment of the car.

In the old days, a different exhaust was often enough to move the harmonics around or get rid of them altogether, but I can understand why that’s not really an option on a brand-new vehicle.