2006 Vauxhall Astra loud crack or clunk from steering

This happens mainly when there's a high load on the steering such as slow manoeuvring in a car park. It mainly happens as the steering changes direction rapidly. It seems to come from the direction of the rack but may be slightly to the offside. It's impossible to make it happen stationary or without the engine running. I've leverd and prized at every joint and am starting to strip suspension components completely to examine before working inwards towards the rack. Has anyone any experience of such noises and is it the sort of noise a rack could produce? Thanks in advance!