Our eight year old Volvo XC90 has developed a low whine from the rear left tyre, which starts at around 30pmh and reaches highest volume at 60mph and is not heard thereafter (don't know if that is just because general cabin noise at motorway drowns it out).

I did reverse into post at low speed recently, but there is no visible damage and the exhaust seems fine. Also had timing belt, auillary belt, tensioners and water pump changed at the last service. Any idea what could be causing the noise?