The 3.62 Quaife sierra diff which came with my 2010 Roadsport has always been an annoyance. It has had a persistant high pitched whine between 40 and 60mph and moderate backlash. I took it to R&R who had a go at it, and made it slightly better, but the whine persisted. It ruined what was otherwise a great car. I tried filling it with all sorts of different snake oil, some of which produced a short lived improvement, but it always ended up being really annoying.
I started toying with the idea of forking out £2000 for the BMW diff as the only way to sort the problem. 1 was kicking myself for not taking Caterham up on their offer of a swap to the BMW unit at reduced price.
As a last ditch effort I bought an old original Ford 3.92 off ebay for £20. It was dirty as hell, but had come out of a sierra with only 65000 miles on the clock. I thought about giving it to R&R to set up but it felt good so I cleaned it up and gave it a lick of aluminium paint. Bought some new oil seals off ebay for £18 and swapped them out, and then swapped it out for the Quaife 3.62.
Amazement!! This diff is a revelation. There's no backlash, no grinding of cogs, and minimal whine between 55 and 60 ( at about 1/4 the previous volume). I suddenly appreciate what a great engine note the sigma has when you can hear it.
It makes you realise that this unit can be quiet. It just needs to have the original internals not reconditioned parts! It can also be quiet in a metric chassis. As an added bonus the 3.92 seems better suited to the car, it makes it feel slightly more lively,
All in all about £50 well spent.