My first Q2 was never noisy (fitted at Autolusso Feb 2008, 18 months before I was employed there), but I have driven others that have made a little noise at slow speeds before (Alfa GT JTD Q2, a couple of them). I don't think it is worth considering.
Q2 vs Quaife, I owned one of each up until a few months back, let me say that any difference between the two is not really worth considering when you are a buyer who currently has a standard diff. Just imagine them as the same thing, and find the best price you can for whichever diff. OK the Quaife has a lifetime guarantee as opposed to 12 months for the Q2, and the Quaife is clearly better made as it is machined from billet, but neither of those facts really matter as I've never known a Q2 to fail and once either diff is fitted inside your gearbox you can't see how much nicer the Quaife looks.
As far as I know, no-one is selling a Q2 kit cheaper than our price for the Quaife kit which is £550 inc VAT. By kit I mean diff, pair of bearings, pair of O-rings and pair of driveshaft seals. In addition you will need gear oil, and possibly a shim of different thickness, but you won't know which you need until you start fitting it. Mostly they are correctly shimmed from factory but some of the Alfa recon gearboxes I've seen have not been shimmed correctly to start with.
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