[philrocks]

Sounds like you have tackled most of the common causes, I too have a rumbling sound on mine from the N/S front, though its more noticeable higher speed. No vibrations in the cabin though (I did have, and that was the front discs, which I replaced).

When my O/S drive shaft went, it was more noticeable because I had lots of vibrations, not sure if the sound might just be its on its way out though? Have you checked for play / boot condition on the driveshaft?

I have also got wider wheels / tyres on the car now, and I feel like road noise is lots louder.