XC60 D5 Resonant Vibration

[stiggles]

Hi

I've recently purchased an XC60 D5. It's a 2013 Model year (pre face-lift) D5 Geartronic AWD, with the 230BHP Polestar upgrade. The car generally drives very smoothly with no weird clunks (perfect shifts on the gearbox etc). It's done 95k.

One thing I've noticed however is that occasionally there's some sort of deep resonant sound somewhere in the drive-train. At speeds of 30-40mph you can provoke by maintaining a light throttle on a slight incline. A deep bassy rumble can be heard through the car. Knock it into sports mode, and it doesn't tend to happen as much.

At speeds exceeding 60-70mph again with light acceleration it feels more like a going over a rumble-strip and can be physically felt & heard through the vehicle. At these speeds sports mode doesn't seem to eliminate it. Lifting off or really accelerating hard does seem to cancel it out.

I've been advised my vehicle is current with all of it's software updates. The only other thing to mention is the tyres do not match front to back with Michelin Pilot Sport 4 SUV's on the front & Continental Sport Contact SUV 5's on the rear. The rears have 4mm of tread - the fronts are brand new).

Am I chasing down a normal characteristic of these cars or should I be looking to get the drive-train checked over? I'm considering putting 4 cross-climates on it so that I've got a full matching set of tyres, but obviously it's a large expensive particularly if they all drive like this!

Thanks

David