I had that once, turned out there was a fire earlier in the day in the area.
Mad me cack myself
on a more serious note...
are all the elecs working? have you popped the bonnet to check if it's coming from the bay? smell each tyre just to see if it gets worse near any of them. Have the bearings been playing up at all? I had my bearings (housing only as the round thingys were no where to be seen!) seize up on the sub axle and didn't realise until I got back from Wolverhampton (110miles
) but there was an aweful smell when I stopped at the services.