thanks lads please bear with me on this one it's quite intermittent, so difficult to pin down. have tried with difflock and no knocking, but when i disengaged the difflock it didn't knock either.
Next day it knocked a couple of times without difflock tried to engage difflock but couldn't catch it in time.
It's only when I'm coasting to a stop and knock it out of gear.
i'll keep you posted.