Not being there to diagnose it exactly - it sounds to me like a loose component vibrating - and not a tyre issue.
The problem with lower pitched noises is they can seem to come from anywhere as they dont attenuate as fast with distance as a high pitched hum.
The good news it though that lower pitched hums come from larger objects vibrating, not smaller ones. (Think longer strings on a harp - they have the lowest tone).
There are some iPhone apps you can get for free that measure volume - "librarian" is one. It has a trigger that sounds an alarm when you exceed an adjustable volume threshold.
I would try using it to measure the hum as you move the phone around the cabin and see if it is louder in one spot.
That way you can narrow down the search for what might be vibrating.
Put pressure on any nearby surface (that can either hold it still or change its effective mass so it vibrates at a different frequency) to see if you can make it go away to confirm you are identifying the root cause.
If you want help - just send a return ticket to Australia for me...
I've never been to Italy.