Wheel bearing is favourite suspect despite the noise not changing, you might find the source if you jack one wheel up at a time and spin it up, though sometimes wheel bearings don't make any noise until they have some weight on them so can be difficult to diagnose accurately, spinning the wheels up by hand will also show up something dragging against a disc.

Have you checked the gearbox oil level, hopefully too new for a gearbox problem but if its sprung a leak and you haven't noticed...