E30 Zone

Please excuse my daft questions but this is my first RWD car. There is a whining from the back end of my 325 convertible when I'm doing over 35mph Is this the Diff? I changed the oil but it hasn't made an difference.

If it is the diff, are they the same throughout the range or specific to my model.

Thanks lads

Got to be bearings in each hub, difficult and expensive.

I wouldn't describe the noise from failed wheel (hub) bearings as a whine - tends to be more of a rumble.
If it's a whine it's almost certainly the diff. input bearing. This is very common on E30s, much more noticeable on rag tops, even more noticeable on RWD cars (where the noise isn't lost in general engine/transmission noise, but isn't necessarily anything to worry about. A whining diff. can work happily for tens of thousands of miles before it gives up completely. It's final demise is likely to be heralded by a considerable loudening of the whine.
Unfortunately, replacing the input bearing on an E30 diff. is not straightforward, and requires specialist knowledge and tools to do properly, so is not a DIY job. We have someone on the zone however, who rebuilds diffs - one of the team, who goes under the name of Gareth.
E30 diffs come in two case sizes, commonly known as small and medium, which are mechanically interchangeable with no problems. However, your car is a 325i, which really needs a medium case diff. for reliability.
There is also the gear ratio of the diff. to consider. IIRC, yours should be 3.9:1, but if I was sourcing a replacement, I'd look for a 3.76 to aid fuel consumption a little.

Thanks Brian for such a comprehensive reply. Answered all my questions

Drop me a PM if you want prices for any diff work, or just to get pointers if you're looking at it yourself