Front diff noise

We have run a 2000 Mk4 2.0ltr for six years and loved it. It has been trouble-free with just our regular servicing and brake jobs to do. But now there is a whine or growl coming from the front left that matches road speed. I assumed it was the bearings, but when I jacked it up the wheel turns silently and wobble-free, as do the other three. A local mechanic took a look and thought it was most likely the front differential. It clicks as you turn it round.

Is this something that can easily be mended? I have contacted three scrapyards and none has a front diff. I am wondering whether this is because it comes as a unit with the rest of the transmission.

Any tips and advice welcome!

It is inside the gearbox and a complete strip down to access, then as new bearings will be needed, the preload of the bearings has to be established which is a time consuming process, and diff's don't tend to growl, wheel bearings and tyres do.