YRV 4trak "humming"

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Hi all, new to the forum with a YRV 4trak, sorry that my first post is a problem post :/
My 84k miles '04 YRV 4trak drives extremely well but the driving experience is spoiled somewhat by what I can only describe as a humming/droning noise from the rear of the car. Its not too noticeable at low speed but as the road speed increases so does the volume.
First thoughts are either wheel bearing(s) or diff. I shall be jacking the car up over the weekend to spin the rear wheels and check for any roughness and rock the wheel, top and bottom, to check for any wear/play.
As far as the possibility of it being a diff issue would anybody be able to tell me how to check/top up the diff oil?
I have seen that on the rear of the diff casing there is a large hex headed plug(?) Is this used the same as a gearbox filler, remove the plug and fill up with oil till it trickles from the hole? Also, what is the capacity of the diff for a drain and refill with new oil?
Any thoughts/input/help would be very much appreciated Smile