Possible Bearing or rear diff problem
 

I have been hearing a slight whirring noise coming from the back of my car for a while now, recent my Brother Inlaw sat in the back and heard it saying it sounded like the left rear wheel. I only rear the noise when travelling around 50-60Mph in 5th or 6th gear. Slower speeds are usually higher engine revs so I hear the engine and over 60 the tires get louder then the noise.

I read on hear that you can check the bearings by jacking up the car taking off the handbrake and wobbling the wheel to check for any play. I did this and both wheels feel solid with no play and no strange noises whilst spinning from the wheel area.
When turning the wheel back and forth there is quite a lot of noise coming from the rear diff and prop shaft. It is only the initial change of direction when you can hear a noise no grinding just the sound of cogs and gears lining up. Is this normal or do I need to look at topping up the fluid in then so it is quite or could there be an issue there somewhere.

Background info. When I bought the car there we two different size tires on the back 225/45/18 and 245/45/18 could that have damage the read LSD or Diff (not sure if they are the same thing)

On a funny note. When you jack up your car and take the handbrake off make sure you put the handbrake back on before you remove the jack stands and lower the car otherwise you will watch your pride and joy roll off the jack and not stop until it crushes the jack under the front tire. Fortunately no damage to the car, jack is a bit scratch but it adds character.

^I drained my diff at 3500 miles and it was full of metal mud, so drain yours, and put in good 75w90 syn like Mobil 1 (about 1.6 quarts).

Next, check the ujoints on the rear half shafts, and main drive shaft.

If no looseness, you've done all the fixing you can do unless you have warranty.

My new 07 6spd auto with SPT's made a pretty loud whine at about 50-60 mph when new.

After broke in with Hankook V12's, I don't hear or notice.

A lot of different cars diff's make noise at that sweet spot (light load).

Thanks for your reply, glad to hear the noise is probably nothing , I will get the oil for the rear diff.