I get this sometimes if I've been driving hard taking the scenic route home. Its just the tyres getting a bit warm from being worked hard. It'll be more obvious if you've done a lot of hard accelerating and braking than if you have just been trundling on cruise at 70. If it does it when you've just been trundling along then I'd check tyre pressures haven't dropped too low. You haven't got Avons by any chance? I've noticed it more with my latest Avons than I previously did with my Pirellis or any other make of tyre that I've had fitted.