Should not be any wiggle room! Before replacing my banana bars they could be rocked back and forth by hand, but I knew the knocking was coming from them way before that. If you are out in the car today, at low speed, with the knocking occurring, slowly apply the brakes and the knocking will completely stop. This is because of weight transfer, the front hunkering down, compressing the arms which means no wiggle room. (My thinking anyway!)
If this is the case you can be pretty certain its the banana's, I was quoted about £130 for just the arms fitment.
There were 2 brand new arms for sale on here at a bargain price, not sure if they are still about!
If not, , brand new banana arms for £125 each, cannot go wrong!