Ominous knocking noise

[PaddyLondon]

MY2000 XKR, 94000 miles. Driving back from London, a knocking sound started from the rear of the car. Over about 10 miles it got louder. It changes frequency a little when the car is driven faster, but the frequency nor loudness of the knock is not proportionate.

The car is "driveable" but the sound makes it sound a very bad idea.

The sound appears under acceleration or maintenance of road speed. Under engine braking ("foot off the accelerator"), the sound is inaudible. Driving in reverse gear, it is inaudible/not present.

A local mechanic put the car on his ramp to inspect. The noise could be heard with drive engaged and the wheels rotating. Using a screwdriver as a listening device, he pinpointed the noise to the differential. He said there had been Oil leaking from the diff, though I have not seen evidence of this on my garage floor. He has proposed to replace the diff, causing my eyes to water.

I'd be very grateful for any advice or stories of similar issues. eg

What other diagnostics could I/should I have done before committing? Are there any interim "sticking plasters".
Should I drive it to and from the mechanics or have it transported.

[Lazlo Woodbine]

I've never been under a car like yours so I don't know what the set up is like but; I'd certainly want to have a look at the diff Oil before considering a new unit. If it comes out burnt or looking like Goldschlager then yes it's not healthy but even then I would expect it to whine or whirr not knock. A knock makes me think more of prop/drive shaft joints or splines, maybe even a wheel bearing, and it's entirely feasible that that noise could be heard in the diff housing what with them being connected. 

EDIT; Just remembered I had an old Volvo with a two piece prop shaft and the joint was supported by a rubber donut. This collapsed on one side and let the shaft knock going forwards but was silent in reverse as it pushed the joint to the opposite (good) side of the donut. Just a thought.

[Old Peter]

Welcome to the club, David.

Ralph has pointed out some possibilities which are well looking at.

Good luck with the problem and hope that th bill is just a little one!

Regards,

Peter.