Our latest Thule/Brink tow bar [2016] comes with instructions to remove it when not in use, and the spate of wear issues LR had with these was largely blamed on them not being removed when not in use.
I was one of those early users [from 2004] when such an instruction was not given, and I left ours too frequently in place, and suffered the locking pin's cam face fretting wear that led to the in-use clanking. Not in itself an immediate danger but both an irritant and in the longer term does become damaging.
I was covered many years down the line by the recall fix, but unfortunately then ended up with their monstrosity of the fixed tow ball/plough unit.
So, with the vehicle's 2016 replacement, the tow ball, religiously is removed when not required; seems to be the cure, along with their very subtle design change.