I've noticed a small pop noise come from my passenger side drive line when I start to reverse. The noise only appears once when reversing for the first time.

Am I right in thinking the CV joint (likely the outer) is worn and should be replaced at some point soon?

I dont get a noise when not in reverse and there's no clicking from the CV when turning the wheel.

It would be worth your while getting someone to drive (in reverse) while you stand near the left wheel and look and listen with the bonnet open. May help pinpoint the exact location of the pop noise.

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ex-NSW Police 1970 MK II Cooper S
VMCI #43

Check torque on the front hub nuts.
Also check the drive flange nuts if it's an S. These work loose

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.