Background drone?

Random question, but does anyone else have a dull background droning noise, fairly low pitched and not loud when driving. It's unrelated to engine revs, sort of increases with speed. When stopped it's not there. I don't think its tyres as it doesn't really change with different road surfaces. My A110 has the standard exhaust.

I think it's always been there since new. I hadn't been out in the car for a while but did 60-70 miles on friday, mix of motorway, a road, b road, not going fast, just normal. Mix of standard and sport mode. Car drives totally fine but once I 'tuned' into the background drone it was there constantly.

It's hard to describe but not a resonance as such, doesn't come and go with speed / revs. I don't think it's wind noise, I don't think it's driveline. The car has 8500 miles on it so it's nothing worn out. I can only think it's a consequence of the aluminium shell and given I hadn't driven the car for a little while it suddenly became obvious. I'm sure it's nothing, but curious to know if anyone else notices something similar?

Julian, are you selling aftermarket audio kits for the A110 then or is this a professional research project on your own car? I'm quite impressed with the focal audio but presumably there is demand out there for what you are doing? I doubt the original A110 engineering brief dwelled heavily on audio performance as a priority.
Just curious!