YEs, tyres are the simplest change to make. They have noise ratings now, so look for the ones with the lowest noise rating. Failing that, sound deadening. It would be a lot of work, but see if you could get the carpets and underfloor sound deadening from a higher-spec model. Anything you can add over the wheelarches in particular would be good. Also make sure the tailgate is pulled down had against its seal. When we first got my daughter's car, the striker on the body was loose and there was a slight gap.