Lee,
I recently replaced the wheel bearing on the back of my 320. If you jack the car up and test for play in the hub by shaking the wheel you won't experience any movement even if the bearing is worn due to the assembly of the bearing. The best thing to do is drive the car at about 40-50 mph and test to see whether the sound goes away or gets quieter when you go round left or right hand bends. When the bearing in question gets loaded it will make more noise. I experienced a lot more noise going roung left hand bends as a result of my offside rear bearing being worn. It is possible that the bearing was damaged when fitted into the hub as it takes some force to fit it.
Another idea to as well as Nick's as to the noise could be a worn diff. This will be a whiring sound a bit like a wind up car as you roll to a standstill. jack the car up and spin the rear wheels with your hands and feel for any excess play in the diff gears.
The pertol tank leak maybe coming from the pipe that joins the two two tanks. It is S shaped and sits about 200mm infront of the axle carrier and about the propshaft. Where it couples to the tanks I have seen these joints rust out before and leak.
Hope this helps,