Just started lately or just started bothering you lately? Maybe you have an exhaust leak or something under the cab.
Otherwise if you have the time you can pull out the seats and carpets and put down some dynamat or resomat or something similar. It makes a massive difference to cabin noise. I know someone on here put it in their Nav, though I haven't yet. I did put it through my HQ Statesman and it is so much quieter and less drummy.
It is a bit of work but there is plenty of info online